test(okf): consume nav-golden fixtures + thin STEG 0 corpus gate

Consume commons' nav-golden fixture class byte-exact from
portfolio-optimiser-commons @ b641741 (nav-golden-hierarchy positive +
nav-golden-escape boundary) into test/nav-golden-corpus/ and wire a thin
gate scripts/check-nav-golden.mjs (+ .test.mjs, 10 tests).

Scope (operator decision, thin): the catalog is the convention owner and
holds no consumer (null konsument), and the shared retrieval skill is
deferred (spec.md §10 Stage 3). So the gate does NOT run a read-context
navigator and does NOT assert the goldens byte-exact. It asserts only what
the catalog owns: each fixture bundle/ is a conformant OKF bundle under the
existing §3 checkBundle (every concept typed + root okf_version), and the
committed expected-read-context.md golden is present and non-empty. Byte-exact
navigator conformance stays a consumer concern (okr STEG 4 / Stage 3 skill).

Fixtures pinned byte-exact (.gitattributes -text); manifest carries source
repo+SHA per the vendoring-provenance rule. Naming: the stale "§5 pts 1-5"
read-context label maps to the settled method-spec §3 Step 1 (commons 9801d35);
no open §5 dependency. commons confirmed b641741 complete + goldens derived
from §3 Step 1 (via coord).

Verified: check-nav-golden 10/10; gate CLI exit 0 (2 fixtures PASS); full
suite 73/73 across the six test files.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NeK9hkxrU9wFPBYGYnSV1V

test/nav-golden-corpus/nav-golden-escape/bundle/index.md

@@ -0,0 +1,12 @@
+---
+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)

test/nav-golden-corpus/nav-golden-escape/bundle/valid.md

@@ -0,0 +1,6 @@
+---
+type: project
+title: "Valid sibling"
+---
+
+The only in-bundle target. A conformant navigator returns exactly this file.