Finish the 0.3.0 renames: bunInstall, testForked, bunBundleFormat, env hooks

[arcaputo3]

Stacked on #21 (agent/bun-unmanaged-deps). Completes the 0.3.0 rename decision: one migration, not two.

What changes

bunInstall is the canonical TypeScript install task. Mill's inherited npmInstall delegates
to it and keeps working (upstream internals resolve it, and so can users). All internal call
sites use the canonical name, so install artifacts move to out/<module>/bunInstall.dest.

Both module kinds share one test entrypoint: testForked.

• TypeScript's test command is a deprecated alias of the new testForked.
• Scala.js' bunTest is deprecated in favor of the inherited testForked — review showed it
  duplicated it exactly: the overridden jsEnvConfig and testLinkTask already put every test
  run on Bun, so bunTest added nothing but a second name.

bunBundleFormat is T[Option[String]] on TypeScript, matching Scala.js. This is the one
rename with no alias path — a type change can't be deprecated gracefully — so it lands as the
documented hard break. Default preserves behavior (Some("esm"/"cjs") from enableEsm); None
lets bun build infer. workerBundleFormat follows it directly.

Environment hooks share one vocabulary. bunToolEnv (install / lock / build subprocesses)
is defined once on BunToolchainModule and used by every runBun call site — previously the
Scala.js, workspace, publish, and workers modules called bunEnv() directly while TypeScript
used a protected bunToolEnv, so overriding the toolchain environment behaved differently per
module kind. TypeScript's bunRuntimeEnv is now public. Scala.js keeps bunJsEnv /
bunJsEnvArgs / bunTestJsEnv unchanged — that's released API for the Scala.js test JS env.

Compatibility story

Old	New	Path
npmInstall (TS)	bunInstall	inherited name delegates, stays usable
test (TS tests)	testForked	@deprecated alias
bunTest (Scala.js tests)	testForked (inherited)	@deprecated alias
bunBundleFormat: T[String] (TS)	T[Option[String]]	hard break, documented with example

MIGRATING-0.3.md gains the new table rows, the bunBundleFormat before/after example, and the
env-vocabulary note. CHANGELOG updated under [Unreleased].

Tests

Integration tests exercise canonical names throughout, plus explicit alias regression coverage:
app.npmInstall must resolve to bunInstall.dest, and the deprecated test / bunTest
commands must keep evaluating until their removal at 1.0. The typescript-bundle fixture's
bunBundleFormat override was migrated exactly as the doc shows. Full suites green.