feat(npm): add Android arm64 (Termux) support

[gustav-fff]

Closes #692

Root cause

@ff-labs/fff-node and @ff-labs/fff-bun declare os: [darwin, linux, win32], so npm rejects Android installs with EBADPLATFORM. Even with npm_config_force=true no binary resolves because TRIPLE_TO_NPM_PACKAGE has no aarch64-linux-android entry and getTriple() throws on process.platform === 'android'. CI already cross-compiles aarch64-linux-android (.github/workflows/release.yaml:182-187, 245-256) but the "Prepare npm package" and "Upload npm package artifact" steps were skipped, and no platform package existed.

Fix

• New packages/fff-bin-android-arm64/ with os: [android], cpu: [arm64].
• Map process.platform === 'android' → linux-android in getTriple() for both fff-node and fff-bun.
• Add aarch64-linux-android → @ff-labs/fff-bin-android-arm64 in TRIPLE_TO_NPM_PACKAGE (both packages).
• Add android to os and the new package to optionalDependencies in fff-node and fff-bun package.json.
• Set npm_package: fff-bin-android-arm64 on the android matrix leg and unskip the prepare/upload steps in .github/workflows/release.yaml so the existing NDK build gets published.

Steps to reproduce

On Termux (Android arm64), on main before this fix:

pi install npm:@ff-labs/pi-fff
# npm error code EBADPLATFORM
# npm error notsup Unsupported platform for @ff-labs/fff-node@0.10.0:
#   wanted {"os":"darwin,linux,win32","cpu":"x64,arm64"}
#   (current: {"os":"android","cpu":"arm64"})

Workaround the reporter found (npm_config_force=true pi install npm:@ff-labs/pi-fff) bypasses the platform check but leaves resolveFromNpmPackage() returning null (packages/fff-node/src/binary.ts:68), so runtime dlopen still fails outside a dev workspace.

After this PR + first release cutting @ff-labs/fff-bin-android-arm64, the same command should succeed without --force, and findBinary() should return the libfff_c.so inside the android platform package.

How verified

• npm run typecheck in packages/fff-node and packages/fff-bun — pass.
• npm run build in packages/fff-node — pass.
• CI android build path already existed and produced target/aarch64-linux-android/ci/libfff_c.so; only packaging wiring changed.
• No runtime verification on-device — @dmtrKovalenko + @joaothallis, please dogfood a nightly on Termux before tagging a release; if ldd/dlopen of libfff_c.so against Bionic surfaces an unresolved symbol, we may need to adjust ffi-rs handling (Termux uses @yuuang/ffi-rs-android-arm64@1.3.2 which npm already selected in the reporter's log).

Automated triage via Gustav. Honk-Honk 🪿

[joaothallis]

Thanks!