feat: add bun module

[TomChv]

A Dagger module for Bun, written in Dang,
modeled on dagger/deno. It models a Bun project
as a typed object graph and maps Bun's toolchain onto Dagger's first-class verbs
rather than wrapping the bun CLI.

Design rationale: design/bun-module.md.

Object graph

• Bun — the pinned toolchain: version, base, install(ctr, musl), and
  lockfile-keyed discovery (projects, project --path).
• BunProject — a single project; cascades source → container → installed and
  carries the verbs.

Verb mapping

The defining constraint: Bun ships no linter, formatter, or typechecker (it
transpiles TypeScript by stripping types), so this module implements only the
verbs Bun actually owns and delegates the rest.

• Checks (@check): test (bun test), audit (bun audit),
  install-check (bun install --frozen-lockfile), plus workspace-wide
  test-all / audit-all / install-check-all.
• Build functions: bundle → Directory!, compile → File! (cross-compile via
  --target); entrypoint defaults from package.json main.
• Primitives: install(ctr, musl), base, container, installed; the
  install cache is a cacheVolume on ~/.bun/install/cache.
• Zero @generate / @up, by design — Bun has no idempotent no-arg
  normalization and no guessable server entrypoint; dependency mutations
  (add/update) and dev servers are intentionally not modeled (see design doc).

Lint/format/typecheck are delegated to specialized modules (Biome/oxlint/ESLint,
Prettier/dprint, tsc), shown in the README's extension example.

Validation

.dagger/modules/e2e installs this module as a dependency and drives it against a
hello fixture plus in-memory workspaces for the failure paths.
dagger --x-release=v1.0.0-beta.7 check → 12 checks pass (clean-project,
compile, bundle, discovery fan-out, subdir-scope, subdir-enclosing, workspace, workspace-subdir,
failing-test, install-into-container, project-lookup).

Discovery keys on package.json + its workspaces field, so a monorepo's
members are enumerated individually (a Bun workspace has one root bun.lock, so
lockfile-keying only found the root); each member installs at its workspace root so
workspace:* cross-deps resolve. It is also cwd-aware, a faithful implementation of
dagger/dagger#13688 — (parentConfigDir + descendantConfigDirs) — so from a
subdirectory the verbs act on the project you're in, shadowing farther-up
projects and skipping siblings. Verified against a real 4-member monorepo repro.

Verified against real images: bun is at /usr/local/bin/bun, glibc on
oven/bun:<v> and musl on -alpine (so install(musl:) copies the matching
build); targets need a C++ runtime (libstdc++), which debian:stable-slim has
and bare alpine does not.

Targets Dagger engine v1.0.0-beta.7; run with dagger --x-release=v1.0.0-beta.7
until that release is the default.