feat: consume prebuilt V8 archives in wheel CI, retire GCP runners

[imfing]

Stacked on #9 (base branch feat/abi3-wheels; retarget to main / merge after #9). This is the final piece of the prebuilt-V8 plan: wheel builds stop compiling V8 and the GCP machinery leaves the repo.

Summary

• Linux wheels build on free GitHub-hosted runners: x86_64 on ubuntu-latest, aarch64 natively on ubuntu-24.04-arm — both inside manylinux_2_28 containers via maturin-action. Expected build time: minutes (just the Rust crate; no V8 compile).
• Prebuilt V8 consumption: the linux job derives the v8 version from Cargo.lock, downloads src_binding_release_{target}.rs from the librusty_v8-v{version} release, and points RUSTY_V8_ARCHIVE (URL) + RUSTY_V8_SRC_BINDING_PATH at the assets. A missing archive release fails loudly with instructions to run the Build V8 archive workflow — the forcing function after any v8 bump.
• aarch64 wheel tests run natively on ubuntu-24.04-arm (the QEMU-in-docker test path is gone).
• Deleted: GCP provision/cleanup jobs, startup.sh, build-manylinux-wheels.sh. (GCP_* secrets/Workload Identity are no longer referenced and can be cleaned up.)
• Added verify-v8-archive workflow (manual dispatch): whole-archive -Wl,-z,defs link test under real glibc 2.28 proving archive linkability — missing symbols must all be first-party Rust FFI (temporal_rs_/rusty_v8_/v8__/v8_inspector__/cppgc__/crdtp__, provided by cargo at the real link); anything else fails. Validated green on the current archive: run 36933430748 — resulting .so requires at most GLIBC_2.28.

Prerequisites already in place

• Build V8 archive workflow + hardened builder images (merged in chore: bump deno_core 0.368.0 → 0.412.0 (v8 142.1.0 → 150.4.0) #10/fix: fall back to clang when image-configured cargo linker is missing #11)
• librusty_v8-v150.4.0 release: all four assets symbol-audited and link-verified under glibc 2.28

Release flow after this merges

1. Tag push → wheels build in minutes on free runners, single cp310-abi3 wheel per platform (with feat: build abi3 wheels with pyo3 abi3-py310 feature #9), tested across Python 3.10–3.14 on both arches.
2. On a deno_core/v8 bump → wheel CI fails pointing at the missing archive → dispatch Build V8 archive once (~1.5h, free) → done.

Verification notes

End-to-end wheel-build verification requires this workflow to run (workflow_dispatch on CI is available post-merge, or trigger via a pre-release tag). The constituent pieces are individually proven: the archive links under glibc 2.28 (verify workflow), RUSTY_V8_ARCHIVE/RUSTY_V8_SRC_BINDING_PATH are honored by rusty_v8's build script (source-verified for v150), and maturin-action mounts the workspace at an identical path in-container with RUST*-prefixed env forwarded (plus explicit -e flags).