ci: cache bun install across matrix (needs + shared cache)

[say8425]

Summary

The matrix from #15 re-ran bun install on all 5 runners (5× network installs). This adds a warm-up install job and shares the bun global cache so the install cost is paid effectively once.

• New install job: checkout → setup-bun → actions/cache (save ~/.bun/install/cache, key bun-<os>-<hash(bun.lock)>) → bun install --frozen-lockfile.
• The 5 check matrix jobs now needs: install and restore the same cache key. With needs, the install job finishes (and populates the cache) before the checks start, so each check's bun install --frozen-lockfile is a cache hit — no network, ~instant.
• Matrix parallelism + per-check status (lint/fmt/typecheck/test/build) is preserved; fail-fast: false unchanged.

Note: needs only orders jobs and does not share the filesystem across runners, so each check still runs bun install — but against a warm cache it's offline + fast. The network fetch happens once (install job).

Test Plan

• YAML parses; name: CI, triggers, concurrency, needs: install, 5-task matrix all intact
• All five commands pass locally (lint, fmt:check, typecheck, test, build)
• CI: install job runs first, 5 check jobs hit the cache and go green

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