disk: bound resident LRU index entries with WithMaxEntries (WS2)

[shreyas-blacksmith]

Summary

• The disk cache's byte budget bounds cached bytes but not entry count: zero-byte blobs charge nothing and tiny blobs at most one 4 KiB block, while each resident entry costs a measured ~268 B of index metadata (key string, entry struct, list node, map slot). A byte-full 10 GiB cache can hold ~2.6M entries, i.e. ~670 MiB of request-independent resident memory.
• Adds an opt-in WithMaxEntries(n) option: inserting a new key past the cap evicts from the LRU tail, exactly like the byte budget but counting entries. The startup scan enforces it too (oldest trimmed during load, like a restart with a smaller max size). Default (n <= 0) is bit-identical to upstream.
• Adds bazel_remote_disk_cache_max_entries_evictions_total to separate count-cap evictions from byte-cap evictions, and an opt-in measurement test (TestLRUEntryMetadataCost) documenting the per-entry constant.

Part of the embedder's memory-envelope program (WS2); stacked on the bounded zstd transcoding branch (#27).

Test plan

• go test ./cache/... ./server/... green
• New unit tests: tail eviction order, zero-byte entries bounded, overwrite does not evict, disabled by default
• New integration tests: disk.New + WithMaxEntries under puts; startup trim of an over-cap cache directory

Made with Cursor

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