docs(examples): nav-golden fixture class — bundle → read-context goldens

New fixture class for the D4 step-0 split (commons-owned; delivered into
catalog's corpus/runner). A nav-golden case is bundle in / expected-read-
context out — the shape that expresses "two conformant implementations MUST
produce an identical read-context" (method-spec §3 Step 1).

- nav-golden-hierarchy/ (positive): depth-first first-seen order, both link
  forms (/ = bundle root vs relative), one segment per level, resolved-path
  dedup, cycle termination, recursive verdict exclusion, missing-index binds
  root alone, flat render.
- nav-golden-escape/ (negative, gate must go red): .. escapes and a
  /etc/passwd root-relative trap; conformant returns only valid.md.

Both expected-read-context.md verified byte-for-byte against a conformant
navigator implementing §3 Step 1; the negative case verified to diverge
under a naive navigator.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VUCuNNvnCkayuUDy2WFgeU

examples/nav-golden-escape/README.md

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+# nav-golden-escape — negative navigation golden (the gate must be able to go red)
+
+Per catalog's requirement that step 0's gate must be able to **fail** (a gate that can
+only pass proves nothing), this negative case makes a conformant navigator and a naive one
+**provably diverge**. It is the counterpart to `nav-golden-hierarchy` (the positive case).
+
+`bundle/index.md` links one valid sibling and three escaping / trap targets:
+
+| Link | Class | Conformant outcome |
+|---|---|---|
+| `valid.md` | in-bundle | returned |
+| `../SHOULD-NOT-BE-READ.md` | `..` escape (the decoy really exists, one level up) | skipped; decoy never read |
+| `../../../../etc/passwd` | deep `..` escape | skipped |
+| `/etc/passwd` | root-relative **trap** | skipped |
+
+The trap is the load-bearing one. Under the ratified rule, a leading `/` denotes the
+**bundle root**, so `/etc/passwd` resolves to `{bundle}/etc/passwd` (no such file → skip),
+NOT the filesystem path. A naive implementation that reads `/` as filesystem-absolute opens
+the real `/etc/passwd` — a path-traversal breach. This case fails for any implementation
+that raises, reads the decoy, or leaks `/etc/passwd` content; it passes only when the
+read-context is exactly `valid.md`.
+
+The malformed-target sub-class (an embedded null byte, other invalid path components) is
+covered by the method-spec §11 **Navigation boundary** seam as a unit test — a literal null
+byte does not belong in a committed text fixture. (See the relocated finding
+`docs/plan/2026-07-20-funn-portfolio-optimiser-nullbyte.local.md` for the concrete
+null-byte reproduction that motivates the seam.)

examples/nav-golden-escape/SHOULD-NOT-BE-READ.md

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+---
+type: project
+title: "Decoy — MUST NOT be read"
+---
+
+This file sits ONE level above `bundle/`, outside the bundle. If its text ever appears in a read-context, the navigator resolved a `..` escape and breached the bundle boundary. It MUST never be reached.

examples/nav-golden-escape/bundle/index.md

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+---
+type: index
+okf_version: 0.1
+title: "Nav-golden — escape / boundary case"
+---
+
+Nav-golden escape case. Every link but the first escapes the bundle or is a root-relative trap; a conformant navigator skips them all, reads none of them, raises nothing, and still returns the one valid sibling.
+
+- [Valid sibling](valid.md)
+- [Parent escape](../SHOULD-NOT-BE-READ.md)
+- [Deep escape](../../../../etc/passwd)
+- [Root-relative trap](/etc/passwd)

examples/nav-golden-escape/bundle/valid.md

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+---
+type: project
+title: "Valid sibling"
+---
+
+The only in-bundle target. A conformant navigator returns exactly this file.

examples/nav-golden-escape/expected-read-context.md

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+Nav-golden escape case. Every link but the first escapes the bundle or is a root-relative trap; a conformant navigator skips them all, reads none of them, raises nothing, and still returns the one valid sibling.
+
+- [Valid sibling](valid.md)
+- [Parent escape](../SHOULD-NOT-BE-READ.md)
+- [Deep escape](../../../../etc/passwd)
+- [Root-relative trap](/etc/passwd)
+
+## project: Valid sibling
+
+The only in-bundle target. A conformant navigator returns exactly this file.