Workspace test-coverage measurement

Standing tooling that measures line coverage across the workspace, so coverage is a repeatable verification artifact rather than a one-off number.

Feature: Workspace test-coverage measurement FEAT_0120
status: draft
is satisfied by: REQ_0991, REQ_0992, REQ_0993, REQ_0994, REQ_0995, REQ_0996, REQ_0997
is refined by: ADR_0134

Motivation. No coverage measurement exists in the repository today — no tool, no script, no CI job. For a project building a safety argument, test coverage is a standing verification artifact: it must be cheap to re-measure and produce comparable numbers run over run, which means the invocation (feature set, exclusions, test-thread discipline) has to be pinned in tooling, not folklore.

Scope. A repeatable local entrypoint (scripts/coverage.sh) wrapping cargo-llvm-cov (LLVM source-based instrumentation, runs on the pinned stable toolchain); a single --all-features measurement run; denominator curation (generated code and dev tooling excluded); terminal, HTML, and lcov outputs; contributor documentation.

Non-goals (deferred until a baseline exists). CI integration (runner choice, artifact upload, Codecov); coverage thresholds or gates; doctest coverage (requires nightly -Z doctest-in-workspace instrumentation; the repo pins stable); per-crate coverage targets for the safety-relevant core (executor, connector) — a later spec question.
Requirement: Coverage entrypoint script REQ_0991
status: draft
satisfies: FEAT_0120

The repository shall provide scripts/coverage.sh that measures line coverage across all workspace crates using cargo-llvm-cov.
Requirement: All-features instrumentation REQ_0992
status: draft
satisfies: FEAT_0120

The coverage run shall compile and test the workspace with --all-features, so feature-gated code is instrumented and the number is comparable to the CI all-features leg.
Requirement: Serial test execution REQ_0993
status: draft
satisfies: FEAT_0120

The coverage run shall pass --test-threads=1 to test binaries, mirroring CI: each Executor builds an iceoryx2 node plus shared-memory segments, and parallel test processes can exhaust /dev/shm.
Requirement: Denominator excludes generated code and dev tooling REQ_0994
status: draft
satisfies: FEAT_0120

The coverage report shall exclude build-script-generated sources (OUT_DIR output) and the xtask crate from the coverage denominator. Coverage of generated output is noise — the generator’s own coverage is the meaningful signal.
Requirement: Report outputs REQ_0995
status: draft
satisfies: FEAT_0120

The script shall emit a per-crate terminal summary, an HTML report, and an lcov trace file, all under target/ (build artifacts, never committed).
Requirement: Missing-tool diagnostic REQ_0996
status: draft
satisfies: FEAT_0120

When cargo-llvm-cov is not installed, the script shall exit nonzero with an actionable install hint instead of failing mid-run.
Requirement: Contributor documentation REQ_0997
status: draft
satisfies: FEAT_0120

CONTRIBUTING.md shall document the coverage entrypoint, what it measures, and where the reports land.