spec-t2y Remove or implement the stale render-cache claim in specs' doc comments Lined Up

What is wrong: specs documents a render cache that does not exist. `service/service.go::Config.Cache` is described (in its own doc comment and referenced from `gitx/read.go::Blob`, `service/read.go::Document.Blob`, `db/store.go` and `core/names.go`) as "the bleve index and the blob-sha-keyed render cache" — but `Service.CacheDir()` has zero non-test callers, and the only actual consumer of `cfg.Cache` is `filepath.Join(cfg.Cache, "index")` for the bleve search index. Four files agree with each other about something that isn't there.

Why it matters: a reader (or a future donor-copy) trusting these comments will believe there is a working blob-sha-keyed render cache and reason about performance or invalidation behaviour that does not exist — the comments are self-reinforcing precisely because four separate files repeat the same claim.

Fix: either implement the described blob-sha-keyed render cache under `Config.Cache`, or (more likely, given `CacheDir()` has no real callers) strip the "render cache" claim from the doc comments in `service/service.go`, `gitx/read.go`, `service/read.go`, `db/store.go` and `core/names.go`, leaving only the accurate description ("the bleve index directory").

Note: confidence reported — this specific item (unlike two other items in the same audit bundle) was not independently re-verified in this pass; confirm `CacheDir()` truly has zero non-test callers before treating the comments as stale rather than the code as unfinished.

Audit: D48 (cross-repo audit, 2026-08-17)