docs(adr): propose Bun runtime and native tooling

Record the proposed decision to run project-owned JavaScript under Bun
with its native test runner, linter, formatter, type checker and HTTP
server, keeping Angular's standard build pipeline, plus the development
spec for the migration tickets.

Refs #289

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/adr/0009-bun-runtime-and-native-tooling.md

@@ -0,0 +1,18 @@
+---
+status: proposed
+---
+
+# Bun Runtime and Native Tooling
+
+VacOptimizer will use Bun as the runtime for project-owned JavaScript in
+development, CI, and production, adopting its native test runner, linter,
+formatter, type checker, and HTTP server even without measured performance gains.
+This deliberately accepts Bun-specific tooling and migration work while retaining
+Angular's standard compiler and build pipeline, including TypeScript 6 for
+Angular compilation alongside `bun check` for standalone TypeScript checking.
+
+Existing behavior, tests, lint rules, coverage reporting, and benchmark budgets
+must be preserved; incompatibilities must be fixed or reported as blockers before
+declaring completion, and a Node.js fallback does not meet the runtime requirement.
+The [development spec](../bun-adoption.md) records the agreed boundaries,
+acceptance criteria, and remaining verification.