Key the backup key-existence cache by raw hash rather than hex string

[rmanibus]

Context

KeyCacheStore (internal/storelayer/keycache.go) holds knownKeys as a
map[string]struct{} over full object keys — chunk/<64 hex chars> and
friends. BackupManager preloads it with every key under chunk/, content/
and node/, so the set is the size of the repository's object count.

Measured at 500k entries:

structure	per entry
map[string]struct{} with hex keys	136 B
map[[32]byte]struct{}	84 B

A 10M-object repository therefore spends ~1.4 GB on the existence set alone
during a backup. Beyond the 38% size cut, [32]byte carries no pointer, so
the garbage collector stops scanning millions of string headers during the phase
where backup is already allocating hardest — likely the larger effect of the
two.

This is not a blind type swap. listedPrefixes and isContentAddressed both do
string-prefix matching to decide whether a key is immutable (and therefore
whether a Put may be elided). That logic has to keep working, and getting it
wrong in the permissive direction means skipping a write for an object that does
not exist.

See docs/caching.md for why this cache is not redundant with PackStore's
catalog — it is the only one that answers negatively.

Goal

The existence set costs materially less memory and GC time on a large
repository, with the content-addressed/mutable distinction preserved exactly.

Scope

• Decode the hex suffix once and key the set by the raw 32-byte digest, carrying
  the prefix separately — either a map per preloaded prefix, or a tagged
  fixed-size key.
• Preserve isContentAddressed semantics: mutable keys such as index/latest
  must still be written through unconditionally.
• Keep the singleflight write-deduplication behaviour unchanged.
• Handle keys whose suffix is not a 64-char hex digest by falling back to the
  current behaviour rather than mis-filing them.
• Add a benchmark quantifying the allocation and GC change at a realistic object
  count.
• Update docs/caching.md's inventory table.

Acceptance Criteria

• Exists, Put elision and Delete behaviour are unchanged, including the
  negative-answer path for a listed prefix
• a test covers a non-hex key under a listed prefix
• benchmark output in the PR shows the allocation reduction
• go test -race ./internal/storelayer ./internal/engine passes
• golangci-lint run ./internal/storelayer/... ./internal/engine/... passes