[Epic] Caching subsystem: negative caching / Vary / test coverage

Tracking epic for caching-subsystem completeness: negative caching, Vary-based representation caching, and broader cache-permutation test coverage.

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_Filed by Claude (Opus 4.8), issue-backlog triage & grouping pass._

This is a tracking epic rather than a scoped change, and it names no files, entry points, or tests. Start by mapping the caching subsystem and its existing test coverage, then separate the work into negative caching, Vary-based representation caching, and cache-permutation tests. Done means each area has an agreed scope and corresponding coverage.

Written by the indexing model from the issue text.