Build: reduce compile, link, and test feedback time

[chrisbbreuer]

Parent: #459
Depends on: #462

Goal

Reduce local compile/link/test feedback time while preserving every correctness, sanitizer, corpus, and evidence gate.

Current cost

Repository guidance records a 4–6 minute test relink after src/ edits, 8–10 minute filtered probe cycles, and roughly 170 seconds for about 1,430 unit tests across ten shards. Compile-time filters still relink the large artifact.

Scope

• Profile Zig compilation, semantic analysis, generated-table parsing, LLVM/codegen/linking where applicable, test discovery, binary size, process startup, and shard imbalance.
• Split or reuse test artifacts by stable subsystems without duplicating production code or losing cross-subsystem/link coverage.
• Add runtime test selection to a reusable binary where Zig permits, preserving deterministic enumeration and leak accounting.
• Improve shard scheduling from measured per-test history and retain per-shard logs/slowest reports.
• Reduce generated-code/table compile cost through owned generator output/layout changes, editing generators rather than generated files.
• Keep one full combined build/link gate that catches integration/ABI drift.

No-workaround rules

• No test deletion/skip, zero-denominator pass, reduced Debug/TSan coverage, stale binary, disabled leak checks, or external build/cache service dependency.

Acceptance

• Reproducible baseline reports wall/CPU/RSS and phase timing for clean/incremental/focused/full/TSan builds.
• Focused source-to-result latency and full local suite latency improve materially on the reference machine.
• Runtime filtering selects exactly the requested tests without relink and reports a nonzero denominator.
• Full unit, ABI, JIT/VM/concurrency, docs, corpus, and CI matrices remain unchanged in semantic coverage.
• Build stays offline and owned under Build: enforce owned local dependencies and offline builds #462.

Foundation

Current test-parallel shard runner and .zig-cache/unit-shards/ logs; tools/unit_test_runner.zig already records slow tests.

[chrisbbreuer]

Landed the runtime-filtering slice in f454ce61.

Evidence:

• The reusable full-root binary discovers 1,463 tests; Fetch Body selected exactly 1 and streaming compilation selected exactly 4.
• Changing between those filters preserved .zig-cache/o/034dd600e289e5c3fa7f8d6fd1608b8c/test, SHA-256 5f1263b86a3f68d76c52d5da0c9152bbe1c4485b92e2e978923e66b5d50d2d98, and its mtime/size.
• The four streaming cases ran 2/1/1 across three shards; all shards reported the same 4-of-1,463 denominator, with 4 passed, 0 failed, and 0 leaked.
• An unknown filter reported 0 of 1,463 and exited nonzero.
• test-jit uses the same runtime mechanism; BunPress manifest verification and the 89-file docs link gate pass.

This completes the runtime-filter acceptance item. Baseline profiling, measured end-to-end latency improvements, and the remaining full semantic matrix stay open.