Resolves the cascading linker failures introduced by the v1.0.69
.cargo/config.toml block that forced -fuse-ld=mold without ensuring
mold was installed on ubuntu-latest runners. Adds the install-mold-linker
composite action invoked by 15 ci.yml jobs and 3 release.yml jobs.

Zero src/ changes; coverage gate (10/10) deferred to CI revalidation.

9/10 local validation gates PASS; gate 10 deferred per operator
authorization (feedback-never-publish-without-explicit-request).