feat: add `runtime-bun` module

[ronak-create]

Summary

Add Bun as an optional runtime target. When selected, generated projects use Bun-specific APIs and config rather than Node.js defaults.

Why

Bun is a drop-in Node.js alternative with a built-in test runner, bundler, package manager, and SQLite driver.
It changes several generated files:

• package.json scripts use bun instead of node / npm
• bunfig.toml replaces some config
• the SQLite module can use Bun's native bun:sqlite instead of better-sqlite3

---

Design consideration

This is the first runtime category module.

The question "which runtime?" logically precedes backend and database choices:

• Bun's native SQLite affects database-sqlite
• Bun's HTTP server affects backend-express and backend-hono

Consider where in the prompt graph this question should live.

---

Files to generate

• bunfig.toml — Bun config
• Patched package.json scripts (bun dev, bun run build, etc.)
• When database-sqlite is also selected:
  • swap better-sqlite3 → bun:sqlite in src/db/client.ts

---

Implementation hints

• Primary surface:

  • configPatches (rewriting scripts)
  • onAfterTemplate hooks (swapping SQLite driver)

• Installer behavior:

  • detect Bun and call bun install instead of npm install
  • check if packages/core/src/installer/ already has runtime detection

• compatibility.advisory (not hard conflict):

  • backend-django
  • backend-fastapi
    → Bun does not run Python

---

Contributor guidance

Refer to CONTRIBUTING.md → "Adding a New Module"

---

Acceptance criteria

• When Bun is selected, package.json scripts use bun throughout
• bunfig.toml is present and valid
• database-sqlite + Bun uses bun:sqlite instead of better-sqlite3
• Installer invokes bun install instead of npm install
• Snapshot tests for Bun + Express and Bun + Hono combinations