The coverage analysis noted there is no mechanism that measures test completeness,
so a source file silently sitting at 0% (untested or dead) is invisible.
Add scripts/check-coverage.sh: it runs the instrumented suite, prints overall
line/region/function coverage and the lowest-covered files, and FAILS if any source
file is at 0% line coverage (the exact signal that found the earlier gaps). It is
opt-in (instrumented build + llvm-cov are slow and macOS-only) and skips cleanly
where xcrun is unavailable; an optional COVERAGE_REGION_FLOOR enforces a floor.
Running it immediately surfaced two more 0% value types, now closed by
CoverageGateValueTypeTests: the HTML tokenizer's `Token` (a custom Equatable) and
the public `Stream.Document` (built by API consumers, never internally). Overall
line coverage is now 92.66% with no source file at 0%, so the gate passes.
Tooling + tests only; +2 tests; full suite green (1570).
