frontier's gen-hash cache never evicts: every t27c rebuild adds ~1860 entries for good -- keep the newest 3 binaries' groups #8928

Part of #8737 (hot reload). This is a defect in my own H1 (#8741).

Problem

• Why it grows: the gen-hash cache key holds the t27c binary's bytes, so every rebuild of t27c writes a fresh entry for each of ~1860 specs. Nothing ever deletes the old ones.
• Measured on this Mac, 2026-10-11 after a day of rebuilds: ~/.cache/t27/genhash holds 14,795 entries, 58 MB. At ~15 rebuilds a day that is about 100 MB a day, without bound, on a machine that has hit 0 bytes free before.

Change

• Group entries by binary: ~/.cache/t27/genhash/<binary digest>/<key>. The key's value is unchanged.
• Evict at the start of each frontier walk: keep the current binary's group plus the newest GEN_CACHE_KEEP_BINARIES - 1 others by mtime, and delete the rest, along with the flat entries of the old layout.
• The rule, gen_cache_keep(is_current, rank_among_others), in specs/verified/gen_hash_cache.t27:
  • the current binary is never evicted;
  • switching between two or three binaries (master, a branch) stays warm.

Acceptance criteria

• gen_hash_cache.t27 has a test that names gen_cache_keep.
• After one walk, at most GEN_CACHE_KEEP_BINARIES groups remain and no flat entries. A warm walk afterwards is as fast as before, and frontier --list is identical.
• Hand-written foreign lines do not grow.

Boundary

specs/verified/gen_hash_cache.t27

User Scenarios

• Given a developer rebuilds t27c multiple times, When frontier runs its gen-hash cache walk, Then only the current binary's cache group plus the newest GEN_CACHE_KEEP_BINARIES - 1 other binary groups remain, and all older groups are evicted.
• Given a developer switches between two or three t27c binaries (e.g., master and a branch), When frontier runs, Then the cache groups for those active binaries are retained and subsequent walks are warm (as fast as before eviction).
• Given the cache previously used the flat layout (entries directly under ~/.cache/t27/genhash/), When frontier runs after the change, Then those flat entries are deleted during the first walk and no new flat entries are created.

Requirements

FR-001: The cache directory structure MUST be changed from flat ~/.cache/t27/genhash/<key> to grouped ~/.cache/t27/genhash/<binary digest>/<key>.
FR-002: At the start of each frontier walk, the cache MUST evict all binary digest groups except the current binary's group and the newest GEN_CACHE_KEEP_BINARIES - 1 other groups by mtime.
FR-003: The current binary's cache group MUST never be evicted regardless of GEN_CACHE_KEEP_BINARIES value.
FR-004: The function gen_cache_keep(is_current, rank_among_others) MUST be implemented in specs/verified/gen_hash_cache.t27 to encode the eviction rule.
FR-005: specs/verified/gen_hash_cache.t27 MUST contain a test that names and exercises gen_cache_keep.
FR-006: After one walk with the new logic, at most GEN_CACHE_KEEP_BINARIES binary digest groups MUST remain and no flat entries MUST exist.
FR-007: A warm walk after eviction MUST be as fast as before, and frontier --list output MUST be identical.
FR-008: Hand-written foreign lines in the cache MUST not grow unbounded.