build(repo): move to bun 1.4.2

[Shironex]

Summary

Move the repo from bun 1.3.14 to 1.4.2, the version showcase-kit already pins and the current release.

• packageManager: bun@1.4.2, the four setup-bun steps (ci.yml twice, docs.yml, release.yml) and the requirement in CONTRIBUTING.md.
• bun.lock stays at lockfileVersion: 1. bun 1.4.2 reads and keeps a v1 lockfile; only deleting it and regenerating writes v2, which here would move 87 resolutions (astro, rolldown, rollup and more). That is a dependency update, not a toolchain bump, so it is left out.
• The only lockfile change is the 13 workspace version fields that bun 1.4.2 syncs from each package's package.json, which were stale after the last release. No resolution changes.

Test plan

• rm -rf node_modules && bun install --frozen-lockfile with bun 1.4.2: clean, no changes
• bun run build, bun run typecheck, bun run test (ESLint 10 and 9), bun run docs:build, bun run --cwd site test
• bun run docs:readmes leaves the tree unchanged
• CI green on the new bun-version