Add musl variants

[Christof Marti (chrmarti)]

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[Copilot]

Pull request overview

Adds Linux musl build/publish variants for the Node-API addon and updates runtime package selection so the facade can load either glibc or musl native packages on Linux.

Changes:

• Add musl native packages for Linux x64/arm64 and wire them into optionalDependencies and CI/publish pipelines.
• Introduce platform.js and update index.js (and smoke tests) to select packages by platform/arch/libc.
• Extend build tooling (cross containers + pipeline steps) and add a small unit test for platform selection.

Show a summary per file

File	Description
README.md	Documents availability/build strategy for musl variants and updated package counts.
platform.js	Adds platform/arch/libc package key selection helper.
package.json	Publishes platform.js, adds platform-selection test script, and adds musl optional deps.
npm/test/smoke.js	Uses new platform selection helper and maps glibc→gnu naming for local addon loading.
npm/test/platform-selection.js	Adds assertions for libc-aware package key selection.
npm/scripts/build-native.js	Adds musl targets, supports cross, and sets per-target RUSTFLAGS for musl builds.
npm/platforms/linux-x64-musl/package.json	New musl x64 native package metadata.
npm/platforms/linux-arm64-musl/package.json	New musl arm64 native package metadata.
index.js	Switches facade selection from ${platform}-${arch} to libc-aware key via platform.js.
deny.toml	Adds musl Rust targets to license deny configuration.
Cross.toml	Pins musl cross images and passes through musl RUSTFLAGS env vars.
azure-pipelines/steps/build-linux-musl-addon.yml	New Azure step to build musl addons via cross.
azure-pipelines/publish.yml	Adds publish entries for linux x64/arm64 musl packages.
about.toml	Adds musl Rust targets to license/about generation configuration.
.github/workflows/ci.yml	Adds CI matrix entries for musl builds/benches and runs platform-selection test.

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• Files reviewed: 15/15 changed files
• Comments generated: 2
• Review effort level: Low