fix(cli): mds build/check exit 1 on a directory whose only .mds files are _-prefixed partials (#387)

[dean0x]

Summary

BREAKING (CLI): mds build <dir> and mds check <dir> now exit 1 when every .mds file in the tree is a _-prefixed partial. The walker collects partials (they are inputs for watch, fmt and lint) but build/check never compile them, so a partials-only tree used to end 0 built, 0 failed with exit 0 — the same silent green pass #204 closed for the empty tree. It now prints <n> .mds file(s) found in <dir> but all are _-prefixed partials; nothing was built (… nothing was checked) on stderr, even under --quiet, and exits 1.

One predicate, two callers: output::partials_only(&[PathBuf]) -> Option<usize> (None for an empty list — the empty-tree arm owns that — or when any non-partial exists; Some(n) otherwise), consumed by a third nothing-to-do arm in run_build_directory and run_check_directory, placed after the empty-tree arm and before the loop, in the same shape as the all-excluded arm.

fmt / lint / watch unchanged. mds fmt <dir> and mds lint <dir> format and lint partials, so a partials-only library is real work for them (pinned by two new tests). mds watch <dir> has no nothing-to-do arm and still starts. A partials library that is only ever imported from elsewhere should not be passed to build/check on its own.

Commits

1. test(cli): RED — predicate scaffold (unconditional None, with a temporary #[allow(dead_code)] because mds-cli is a bin-only crate and the scaffold has no non-test caller yet) + unit test partials_only_answers + five integration tests in tests/dir_build.rs: build partials-only → exit 1; build --quiet still emits the diagnostic; check partials-only → exit 1; fmt and lint pinned unchanged. RED evidence: 6 tests run: 2 passed, 4 failed (the four behaviour tests failing on Some(0) vs expected Some(1) / None vs Some(1)).
2. fix(cli) — predicate body, the two arms, the build.rs doc block enumerating the three diagnostics; the #[allow] comes off. The interpolated count is named partials_only_count and registered in tests/print_discipline.rs ALLOWED_UNSANITIZED for both call sites (integer counter, same convention as the sibling arms); the path goes through safe_path.
3. docs — CHANGELOG BREAKING bullet as the sibling of build: empty directory build exits 0 — silent success hazard in CI #204, spec §7.2 (new bullet) / §7.3 (three nothing-to-process exits) / §7.9 (exit-1 row), README.

Gates (local, on 344f037)

• cargo fmt --all -- --check clean; cargo clippy --workspace --all-targets -- -D warnings clean; cargo clippy -p mds-cli --all-targets --features startup-race-probe -- -D warnings clean
• cargo nextest run -p mds-core -p mds-cli: 2361 passed (mds-core 1465, mds-cli 896; +6 vs main)
• cargo test --doc -p mds-core: 53 passed; cargo +1.88 check -p mds-core -p mds-cli clean
• npm run test:gates: 212 pass / 0 fail; node scripts/verify-no-control-bytes.mjs ✓ (563 files)
• Added-lines ledger-id check and release-surface diff check: both empty, each with a planted positive control
• Behaviour smoke (debug binary, scratch dir): build src/ → exit 1 with the diagnostic; build --quiet src/ → exit 1, same stderr; check src/ → exit 1, nothing was checked; fmt src/ → exit 0, 0 formatted, 1 unchanged, 0 failed; with real.mds added, build src/ --out-dir out/ → exit 0, out/real.md present, out/_only.md absent

Snyk code scan unavailable (MCP ENOENT); the security/snyk CI check is SCA-only.

Not a release-surface PR. The mds-cli feature knowledge base refresh is deferred to the D2 docs bundle.

Related Issues

Closes #387