feat: add cross-compile build scripts and smoke tests [2/4]

[wyattjoh]

Summary

#43 sets up the monorepo with a wrapper shim that resolves platform-specific binaries, but we can only build a binary for the local machine today. To actually ship npx clerk, we need to produce binaries for every platform users run on.

This PR adds the cross-compilation pipeline:

• scripts/build.ts -- compiles the CLI via bun build --compile for 8 platform targets (darwin-arm64, darwin-x64, linux-arm64, linux-arm64-musl, linux-x64, linux-x64-musl, windows-x64, windows-arm64), with binary format verification using file output
• scripts/releaser/targets.ts -- single source of truth for target definitions, Bun target mappings, and verify patterns
• build-binaries.yml -- reusable CI workflow that matrix-builds all targets on the appropriate runner OS
• smoke-test.yml -- reusable CI workflow that runs the compiled binary on native runners to catch runtime issues (e.g. missing shared libraries) before publishing

Also extends root lint/format scripts to cover the new scripts/ directory and adds *.bun-build to .gitignore.

Merge instructions

This is part of a 4-PR stack (#43, #44, #45, #46). After squash-merging this PR:

git fetch origin main
git rebase origin/main wyattjoh/release-infra
git push --force-with-lease origin wyattjoh/release-infra

Then merge #45, and repeat.

Test plan

• CI passes: format:check, lint, build, test
• bun run build:compile:all completes for the local platform target
• Review build-binaries.yml matrix against targets.ts definitions

[wyattjoh]

PR Stack — Supersedes #31

	PR	Description
1	#43	Monorepo restructure
2	#44	Cross-compile build + smoke tests — ← this PR
3	#45	Release infrastructure (changesets)
4	#46	Release documentation + LICENSE

Review in parallel, merge sequentially 1→2→3→4.
Squash-merge each PR, then rebase the next branch onto the updated base before merging it.

[wyattjoh]

Replaced by #47 (base branch was deleted on merge)