ci: wire specs/ci/affected.t27 -- t27c use-edges + test only the affected specs #7565

Problem

specs/ci/affected.t27 (#7529) decides which specs a change can affect -- the changed specs plus every spec that imports one through use, transitively -- but nothing feeds it and nothing consumes it. No tool prints the use graph as repo-relative paths (t27c graph keys nodes by module name and drops specs that do not parse), so every spec PR still pays for the whole corpus.

Goal

Wire the selector into the build: t27c use-edges prints importer<TAB>imported from the same resolver t27c compiles with (use_resolve::use_targets, no second resolver), and a CI step feeds the diff, the base-and-head edge union and the corpus into gen/c/ci/affected.c, then tests only the selected specs.

Acceptance

• t27c use-edges --specs-dir specs prints one edge per resolved use line, sorted, repo-relative.
• spec-guards selects with affected.c (no selection logic in YAML or Rust), falls back to RUN_ALL exactly where affected.t27 says, and runs t27c test-report on the selected specs only.
• Measured: edge count, selection size and wall time on a spec-only change vs the full corpus.

Boundary

bootstrap/src/main.rs, bootstrap/src/use_resolve.rs, .github/workflows/spec-guards.yml. No change to specs/ci/affected.t27 semantics, suite.rs, corpus-ratchet.yml or the ratchet ledgers. Epic #6655 speed track, follows #7529 / #7548.

User Scenarios

• Given a spec-only change in specs/example.t27, when the CI runs spec-guards, then only specs/example.t27 and specs that transitively use it are tested.
• Given a change to a shared spec imported by many specs, when t27c use-edges --specs-dir specs runs, then it prints every resolved use edge as importer<TAB>imported with repo-relative paths, sorted.
• Given a change outside specs/ (e.g., compiler code), when the affected selector runs, then it falls back to running the full corpus exactly as specs/ci/affected.t27 prescribes.

Requirements

• FR-001: t27c use-edges --specs-dir <dir> MUST print one line per resolved use statement as importer<TAB>imported using repo-relative paths, sorted lexicographically, using use_resolve::use_targets from the compiler's resolver.
• FR-002: The use-edges subcommand MUST be added to bootstrap/src/main.rs and delegate to use_resolve.rs without introducing a second resolver.
• FR-003: .github/workflows/spec-guards.yml MUST invoke t27c use-edges, feed its output plus the diff and corpus into gen/c/ci/affected.c, and run t27c test-report only on the selected specs.
• FR-004: The CI step MUST fall back to RUN_ALL (full corpus) exactly where specs/ci/affected.t27 dictates, with no selection logic in YAML or Rust.

Success Criteria

• t27c use-edges --specs-dir specs exits 0 and prints at least one importer<TAB>imported line for the current corpus.
• gen/c/ci/affected.c compiled and invoked by the workflow selects a subset of specs on a spec-only change; the selection size is < 100% of the corpus.
• .github/workflows/spec-guards.yml run on a spec-only PR completes in less wall time than the full-corpus baseline (measured in seconds).
• Edge count, selection size, and wall time are printed as workflow artifacts for a spec-only change vs the full corpus.