ci: warm test-bench cache on push to main

[anacrolix]

Problem

test-bench only triggered on pull_request, so there was never a base-branch cache on main. GitHub's cache isolation allows workflow runs to fall back to the base branch — but not to other PRs or branches. Without a main cache:

• Every new PR's first push starts completely cold (full module download + recompile)
• Merge-queue runs (merge_group) also start cold — they can read from main but not from other PRs
• Caches saved during merge-group runs are stored under the temporary gh-readonly-queue/... branch and become inaccessible once the merge group completes; they never contribute to main

Fix

Add push triggers for main and release/**. After each merge, a cache-warming run compiles all test binaries and saves them under the main branch scope. Future PR first-pushes and merge-queue runs restore from that warm cache.

On non-pull_request events, the benchmark execution step is replaced with go test -run=^$ ./... — this compiles every test binary (producing identical GOCACHE entries) but runs nothing. The job always succeeds on main because correctness is already guaranteed by the merge queue before the commit lands.

Also adds workflow_dispatch (required by project guidelines) and documents the cache isolation model and compile-only warming pattern in .github/README.md.