feat: build and test musllinux wheels for Alpine/musl platforms

[imfing]

Why

rusty_v8 ships musl prebuilts since v150.2.0 — and the deno_core 0.412 bump (#10) landed us on v150.4.0, just past that threshold. musl has no glibc-style symbol versioning, so the entire problem that forces our self-hosted gnu archives doesn't exist here: upstream assets are directly consumable, making Alpine support a pure CI addition.

flowchart LR
    GNUREL[("librusty_v8-v{ver} release (ours)")] -.->|RUSTY_V8_ARCHIVE| GNU[linux: manylinux_2_28 wheels]
    UPREL[("denoland rusty_v8 release")] -.->|auto-download| MUSL[linux-musl: musllinux_1_2 wheels]
    GNU --> T1[test 3.10-3.14 x 2 arches]
    MUSL --> T2[test 3.10-3.14 x 2 arches, python:alpine]

Loading

Mechanism

linux-musl job (free runners, x86_64 + native aarch64):
    RUSTFLAGS = -C target-feature=-crt-static   # ext modules need dynamic musl
    maturin build --manylinux musllinux_1_2     # rusty_v8 fetches upstream musl .a itself

test-wheels-musl: docker run python:{3.10..3.14}-alpine pytest
    # via docker, not `container:` - node-based actions can't execute on musl

Blast radius

Area	Impact
CI.yml	+2 jobs: linux-musl (2 wheels), test-wheels-musl (10-job matrix); release gains deps
Release artifacts	3 wheels → 5 (adds musllinux_1_2_{x86_64,aarch64}, cp310-abi3)
verify-v8-archive.yml	Matrixed: gnu leg (our archive) + 2 musl legs (upstream prebuilts)
docs/quickstart	Alpine/musl: unsupported → supported
gnu pipeline, runtime code, archive workflow	Untouched

Evidence

• Link verification: upstream musl prebuilts pass the whole-archive -z,defs test inside musllinux_1_2 containers, both arches — run 37123820138
• Full CI from this branch green across all 27 jobs: run 37123973126 — musl wheels in ~2m15s, 302 tests passing on every python:alpine combination

Also in this PR (housekeeping)

Change	Why
Dedup: .github/scripts/v8-version.sh	The Cargo.lock v8-version parse existed in 4 places
build-v8-archive.yml matrix simplified	Target triple + Dockerfile now derived from arch name
Comments tightened across .github/	glibc-drift story told once (Dockerfile.x86_64), one-liners elsewhere
Drop dtolnay/rust-toolchain from macos job	rust-toolchain.toml + preinstalled rustup already pin the version
upload-artifact v4→v7, download-artifact v6→v8, stray checkout v4→v5	v4 targets deprecated Node 20 (CI annotation)
Drop PyPy trove classifier	abi3 wheels serve CPython only; PyPy would be sdist-only and is untested

Each increment validated by branch dispatches of CI + verify (all green).