perf: build-mds.test.ts 18 spawnSync calls per test run (hoist to module scope)

[dean0x]

Problem

tests/build-mds.test.ts runs 18 spawnSync full MDS build invocations in separate describe blocks, one per command file being tested. This causes suite-runtime sensitivity: total test time swings from 15s (quiet machine) to 10min+ (load).

Current structure:

• 18 describe() blocks
• Each describe() invokes full npm run build:mds via spawnSync
• Sequential execution × 18 = 18× build time in test suite alone

Context

• Pre-existing pattern identified as suite-runtime sensitivity blocker
• Related to suite timing variability (noted in memory context)
• Opportunities to consolidate MDS build setup

Solution

1. Hoist MDS build to module scope — run single npm run build:mds before all tests
2. Cache build output — all 18 describe blocks read from cache, zero rebuild
3. Verify incremental — add a smoke test to detect stale cache (rebuild on source change)
4. Measure — compare suite time: 18× individual builds vs 1× shared + 18× cache reads

Acceptance Criteria

• Single module-scope MDS build before test suite
• Cache invalidation on src/assets/commands/ changes detected
• All 18 tests pass using cached build output
• Suite runtime reduced to <20s on typical machine
• No timing flakiness under load (test passes consistently)
• Change documented as performance optimization in code comments

Related

• PR feat: productionalize skill/rule shadowing #260 (feat/manifest-compliance) resolution-summary.md, Fix Separately section
• Related: tests/build-mds.test.ts (pre-existing pattern, suite-runtime sensitivity)