Skill: verify cache dependency from the log

[koriym]

A skill that answers "is the cache dependency actually working?" by reading a session, rather than by reasoning about the code.

Why a skill

The failure is a set-intersection that nobody can see: a parent is stored under the tags of the children it embeds, a write invalidates some tags, and if the two sets do not meet, the parent keeps serving stale content. Nothing throws. docs/reading-the-log.md states the rule; this skill applies it.

Procedure

1. Install DevQueryRepositoryLogModule (recording is off by default), then exercise the flow: read the parent, write the child, read the parent again.
2. Read the resulting sessions and check, in order:
   • the cold read recorded depends_on edges for every embedded child — a missing edge is the bug, before any tag comparison
   • the parent's save_* tags include each child's URI tag
   • the write's session contains an invalidate that is not marker-preceded (a pre_write_cleanup immediately before it makes it the writer clearing its own entry, not a bust)
   • that invalidate's tags intersect the parent's stored tags
   • the second parent read is a cache_miss, not a hit
3. Report the first link that breaks, with the two tag sets side by side.

Acceptance

• Distinguishes the four ways this fails, because the fix differs: no dependency edge recorded; edge recorded but the tag absent from the parent's save; a bust that only cleaned up before its own write; tags that never intersect.
• Distinguishes cdn: skipped (no purger configured) from purged and failed — a local pass with a failed CDN purge is still stale at the edge.
• Says explicitly when the log cannot answer: with ProdQueryRepositoryLogModule the healthy read that stored the parent may have been dropped by the retention policy, leaving only the purge side of the intersection.

Notes

demo/run-dependency.php is a working example of the whole flow and its expected log; tests/CACHE_DEPENDENCY_TESTS.md documents what the test suite already pins.