[CTX-0074] chore(test): framer fuzz-smoke harness, seeds and pinned CI job

[Xuepoo]

Closes #124.

Task: CTX-0074. Independent review verdict: APPROVE (PX-0342).

Scope (new files only + one CI edit):

• tests/framer-fuzz-smoke.test.ts (sha256 80f01703, 28 tests)
• crates/devtools-client/tests/framer_fuzz_smoke.rs (sha256 fe15c97c, 20 tests)
• tests/fixtures/fuzz/README.md (sha256 51c0e834)
• tests/fixtures/fuzz/framer-seeds/vectors.json (sha256 a6f77054) + 5 bins, byte-identical Rust mirror (diff -r clean)
• .github/workflows/ci.yml +27 fuzz-smoke job (pinned, read-only, offline)

src/transport.ts and tests/transport.test.ts untouched.

Evidence:

• bun full 391/391 pass; scoped 28/28
• cargo offline full 79 lib + 20 integration pass
• tsc noEmit clean; actionlint clean; prettier clean; oxlint clean; markdownlint 20 files clean; clippy -D warnings clean
• Budgets: local 60s/target 256MiB cap; CI 5min/1GiB single-threaded deterministic (--test-threads=1, CARGO_NET_OFFLINE, --locked/--offline)
• Pinned engines: checkout v7.0.1, setup-bun v2, bun 1.4.2, rust 1.98.1, actionlint 1.7.12

gitleaks on HEAD commit: no leaks (one pre-existing finding in parallel-lane tests/tracing.test.ts, untouched here).