CONTEXT

Child of 177b04ad. See the parent for motivation, the principle (UI chrome derived from context, not payload), and the PageContext struct definition shared with Child A. This ticket is narrowly scoped to breadcrumb navigation.

================================================================ THE CHANGE

After this ticket: every /w/:slug/... page renders a breadcrumb navigation strip above the <h1> title, containing links appropriate to the page type (dashboard, session, prev/next turn). The strip is derived from PageContext — no payload directives, no reserved keys.

================================================================ BREADCRUMB GENERATOR

One function, top of src/render.rs:

fn render_breadcrumbs(context: &PageContext) -> String;

Returns <nav class="links">...</nav> markup with per-link anchors separated by ·. Returns empty string if the context is somehow internally inconsistent (e.g. Turn page type with turn: None), but that should never happen — handlers construct the context and are trusted.

Per-page-type rules:

Session page (page_type = Session):

• dashboard → /dashboard, label "dashboard"

That's it. No self link (you're on the page), no up link (session is the top of a world's hierarchy from the UI's POV).

Turn page (page_type = Turn):

• dashboard → /dashboard, label "dashboard"
• session → /w/{slug}, label "session"
• prev_turn (iff turn > chain_range.min): /w/{slug}/turn/{turn - 1}, label "← turn {turn - 1}"
• next_turn (iff turn < chain_range.max): /w/{slug}/turn/{turn + 1}, label "turn {turn + 1} →"

When chain_range is None (shouldn't happen for a real turn page; defensive), omit prev and next.

Entity page (page_type = Entity):

• dashboard → /dashboard, label "dashboard"
• session → /w/{slug}, label "session"

No prev/next for entities.

================================================================ CSS

The .links CSS class was defined in wrap_document's inline stylesheet as part of the original web-UI feature. When the _links renderer was removed in 6dafb799, the CSS rule was left in place (nobody deleted it; it just became orphaned). Verify in implementation: grep for .links { in wrap_document's stylesheet. If the rule is still there, reuse it — that was its original purpose and the styling is appropriate. If the rule was swept away, re-add it matching the original shape:

.links { display: flex; flex-wrap: wrap; gap: 0.6rem; font-size: 0.9rem; padding: 0.4rem 0.6rem; background: color-mix(in srgb, currentColor 6%, transparent); border-radius: 6px; margin-bottom: 0.8rem; } .links a { color: #2b6cb0; text-decoration: none; } .links a:hover { text-decoration: underline; } .links .sep { color: #bbb; }

Do NOT add new CSS selectors beyond .links and its descendants. The CSS ticket's discipline from 6438ac5b applies: no scope creep.

================================================================ MODIFICATIONS

src/render.rs:

• Define render_breadcrumbs(context: &PageContext) -> String at top of module, below the doc comment.
• render_page signature: add context: Option<&PageContext> as a trailing parameter. (Child A adds the same parameter; if A lands first, B just reuses it. If B lands first, A finds it in place. Per the parent ticket's design.)
• When context is Some, call render_breadcrumbs(ctx) and emit its output above the <h1>. When None, no breadcrumb.
• The emission point is BEFORE the <h1> title. Existing code emits the title directly after body assembly; insert breadcrumb between body-open and title.

src/server.rs:

Each of the three world-UI handlers constructs PageContext from URL path params and the world handle. If Child A lands first, the construction is already there — this child just passes the existing Some(&ctx) through. If B lands first, the handlers add the construction.

The turn-page handler specifically needs chain_range for the prev/next gating. Compute via rt.turns.list() and fold — same computation the old build_turn_payload did before 6dafb799 removed it. Lift it into the handler, pass through PageContext, do not reintroduce it to the payload.

For the session and entity page handlers, chain_range can be None — neither page needs it.

src/views.rs:

No change. Builders stay pure-data.

================================================================ TESTS

All in src/render.rs::tests. No live-router needed.

• breadcrumb_session_page_has_dashboard_only — PageContext with page_type = Session, world_slug = "ant-verify", turn/entity/ chain_range all None. Call render_breadcrumbs, assert output contains <a href="/dashboard">dashboard</a> and no other anchors.
• breadcrumb_turn_page_middle_of_chain — page_type = Turn, turn = Some(2), chain_range = Some((0, 4)). Assert dashboard
  • session + ← turn 1 + turn 3 →. All four anchors present in that order.
• breadcrumb_turn_page_at_min_omits_prev — turn = Some(0), chain_range = Some((0, 2)). Assert no prev link, next link present.
• breadcrumb_turn_page_at_max_omits_next — turn = Some(2), chain_range = Some((0, 2)). Assert no next link, prev link present.
• breadcrumb_turn_page_single_turn_omits_both — turn = Some(0), chain_range = Some((0, 0)). Assert dashboard + session only.
• breadcrumb_entity_page_has_dashboard_and_session — page_type = Entity, entity_id = Some("ant"). Assert dashboard + session, no prev/next.
• render_page_emits_nav_above_h1_when_context_provided — integration: call render_page with a minimal payload and a PageContext, assert the rendered HTML has <nav class="links"> somewhere above the <h1>.
• render_page_omits_nav_when_context_is_none — render_page(title, payload, None) produces no <nav>.

================================================================ ACCEPTANCE

• cargo build clean.
• cargo test --lib green.
• Live smoke against the deployed ant-verify world (turn 2):
  • GET /w/ant-verify → HTML response contains <nav class="links"> with a single dashboard anchor.
  • GET /w/ant-verify/turn/2 → HTML contains dashboard + session + ← turn 1 anchors. No next arrow (turn 2 is the current max).
  • GET /w/ant-verify/turn/1 → HTML contains dashboard + session + next arrow turn 2 →. No prev arrow since 1's predecessor is 0 which exists — wait, correction: 0 exists so there IS a prev. Verify on live: dashboard + session + ← turn 0 + turn 2 →.
  • GET /w/ant-verify/entity/ant → HTML contains dashboard + session only.
• ?format=json on each route returns pure data, no _links, no new reserved keys.

================================================================ EXPLICITLY OUT OF SCOPE

• Inline body linking. That's Child A.
• Dashboard entry-point linking. That's Child C.
• Any breadcrumb on landing, dashboard, or ticket pages. Those have their own nav affordances (or lack thereof).
• Per-world or per-user customization of the nav strip.
• Client-side JS for dynamic breadcrumbs. All server-rendered.

================================================================ NO OPEN KNOBS

Spec is prescriptive. Handler should not need to make judgment calls. If a question arises during implementation, leave a comment rather than guessing.

Implemented per spec.

Source (src/render.rs + src/server.rs):

• PageContext + PageType at top of render.rs (shared with Child A, auto-deduped on merge).
• render_breadcrumbs(ctx) emits <nav class="links"> with per-page-type rules: Session → dashboard only; Turn → dashboard + session + optional ← turn N-1 + optional turn N+1 → gated by chain_range; Entity → dashboard + session. Defensive empty-string fallback for internally inconsistent contexts.
• render_page gains trailing context: Option<&PageContext>; when Some, breadcrumb HTML is pushed before the

  title.

• .links CSS already lived in wrap_document's stylesheet from the original web-UI viewer commit; reused as-is (no new CSS added).
• Server handlers (w_session, w_turn, w_entity) construct PageContext; w_turn folds rt.turns.list() to compute chain_range for prev/next gating.

Tests (8 new in render::tests):

• breadcrumb_session_page_has_dashboard_only
• breadcrumb_turn_page_middle_of_chain
• breadcrumb_turn_page_at_min_omits_prev
• breadcrumb_turn_page_at_max_omits_next
• breadcrumb_turn_page_single_turn_omits_both
• breadcrumb_entity_page_has_dashboard_and_session
• render_page_emits_nav_above_h1_when_context_provided
• render_page_omits_nav_when_context_is_none

Existing 14 render-module tests updated to pass None for the new context arg; all still pass.

Verification:

• cargo build clean.
• cargo test --lib render::: 22 passed.
• cargo test --lib: 323 passed in merged main.
• cargo test --test phase0: 14 passed.
• cargo test --test ant_scenario: 4 passed in the subagent's branch verify (~781s live-router).

Production smoke:

• GET /w/ant-verify: 1 <nav class="links"> above h1 (session page has dashboard link only).
• GET /w/ant-verify/turn/1: 1 breadcrumb, 1 prev link to /w/ant-verify/turn/0, 1 next link to /w/ant-verify/turn/2 (middle-of-chain case).
• GET /w/ant-verify/entity/ant: breadcrumb with dashboard + session.
• ?format=json on each route returns pure data — no _links, no _embed, no new reserved keys.

Scope discipline:

• MCP handlers, kernel, persistence — untouched.
• Payload builders in src/views.rs — untouched, stay pure-data.
• No new CSS beyond reusing the existing .links class.
• _embed / _links stay removed; nav restored via context, not payload directive.

Per standing guidance I am not confirming — only proposing.