t27c frontier: a cold walk fills the gen-hash cache with parallel workers before deciding (hot reload H10) #8846

Part of #8737 (hot reload); builds on H1 (#8741) and the suite pool (#8837, #8840, #8845).

Problem

• When the walk is cold: the H1 cache key holds the t27c binary's bytes, so after every t27c rebuild, and on every CI run (spec-guards runs frontier --list on the base), t27c frontier regenerates all four backends of every spec, one spec at a time.
• Measured: 55-68 s at load ~20 earlier today; 16.5 s at load ~7 now.

Change

• A helper: gen_cached(p, tc) holds the four lines that read, compute and store a spec's gen hashes. They are moved out of decide() unchanged, and decide() calls the helper.
• The warm-up: before the decision walk, par_in_order (suite_jobs() workers with 8 MiB stacks, t27c suite runs the in-process phases 1a2-1a7 through the parallel pool, workers on an 8 MiB stack -- 15.9 s -> 7.4 s (Closes #8841) #8845) calls gen_cached for every spec. decide() then finds every entry.
• Unchanged: decide(), the cache key and the rules gen_cache_use and gen_cache_store.
• Why it is safe: compute_seal_hashes reads and generates only. It writes no file and spawns no process, and the compiler keeps no global state but an atomic key counter (audited in t27c suite: run the in-process phases 1a2-1a7 through the parallel pool, workers on an 8 MiB stack (#8838 follow-up) #8841).

Acceptance criteria

• frontier --list from a cold cache prints the same output with T27_SUITE_JOBS=1 and with the default, and fills the same number of cache entries.
• A cold walk is at least 2x faster with the default on this Mac.
• gen_hash_cache.t27 and suite_jobs.t27 name the warm-up.
• Hand-written foreign lines do not grow.

Boundary

specs/verified/gen_hash_cache.t27

User Scenarios

• Given a cold gen-hash cache and a rebuilt t27c binary, when t27c frontier --list runs with the default T27_SUITE_JOBS, then the warm-up phase populates the cache in parallel using par_in_order before the serial decide() walk begins.
• Given a cold gen-hash cache, when t27c frontier --list runs with T27_SUITE_JOBS=1 and with the default jobs setting, then both executions produce identical output and fill the same number of cache entries.

Requirements

FR-001: The helper gen_cached(p, tc) MUST extract the four lines that read, compute, and store a spec's gen hashes from decide() unchanged, and decide() MUST call this helper.
FR-002: Before the decision walk, a warm-up phase MUST invoke gen_cached for every spec using par_in_order with suite_jobs() workers (8 MiB stacks).
FR-003: The cache key, gen_cache_use, and gen_cache_store rules MUST remain unchanged.
FR-004: specs/verified/gen_hash_cache.t27 MUST name the warm-up behavior in its specification.
FR-005: specs/verified/suite_jobs.t27 MUST name the warm-up behavior in its specification.
FR-006: Hand-written foreign lines in the codebase MUST NOT grow as a result of this change.