ci: migrate to the shared fleet workflow

[h4x0r]

Replaces eleven hand-rolled jobs with the shared workflow and publishes ci / All checks. docs.yml and release-plz.yml untouched.

Nothing is left beside the gate. Every job this repo ran is now a job inside the reusable workflow — which matters, because a sibling of the uses: call publishes its own context that branch protection does not require: it reddens a PR and blocks nothing.

was	now
fmt, clippy, test, msrv, deny, vet, secrets, docs	shared equivalents
coverage	coverage-gate: strict + coverage-metric: both
package	package-check — now a gated job in the shared workflow
fuzz-check	fuzz-build
geiger	deleted — see below

Coverage: the strongest posture in the fleet

Strict on lines AND functions, where 47 consumers gate on lines alone. That is this crate's actual invariant — every monomorphized instance exercised, not merely every line touched by some instantiation. Line coverage cannot see a generic reader instantiated for a type nobody calls: llvm-cov emits one FN record per instantiation but one DA record per source line.

Measured at 100% on both, and verified by running the shared gate's own script against this repo's lcov rather than assuming it.

The coverage-metric input exists because of this crate. Before it, migrating here meant silently dropping the function half of the gate.

geiger is not carried over

It was cargo geiger 2>&1 || true under continue-on-error: true — unable to fail twice over. The real control is the shared Unsafe lint audit, which asserts every member effectively forbids or denies unsafe_code — the part the compiler cannot tell you — and it is inside the gate.

The gitleaks install goes too: it queried releases/latest, which rate-limits on shared runners and leaves the version empty. The shared job pins 8.30.1.

Pre-flight

Reading each command's own exit code rather than a pipeline's: fmt · clippy -D warnings · tests · deny · vet · cargo +1.75 test all clean, and the packaging check confirms README.md and LICENSE both reach the published crate.