feat(bun): unlock Phase C selective plugin reload via bun#39426

[ebowwa]

Summary

Unlocks Phase C — selective plugin reload on Bun — using the public-API primitive fixed by our upstream PR oven-sh/bun#39426 ("keep a file:// URL's query in module keys", fixes #21346).

Before #39426, import(url + "?v=N") returned the same cached module instance for every query on Bun, so per-module reload via public APIs was impossible — the reason selective HMR was deferred to Phase C in docs/BUN_COMPATIBILITY.md. @cordisjs/plugin-hmr gets this capability on Node only through --expose-internals module-loader internals, which Bun does not expose.

With the fix, the whole primitive is two public calls:

const mod = await import(`${pathToFileURL(pluginPath).href}?gen=${n}`) // fresh instance
const fiber = await root.plugin(mod)   // activate under the root
await previous.dispose()               // graceful swap — old disposers run,
                                       // old timers stop, root keeps running

What's included

• tests/bun/selective-reload.spec.ts (+ fixtures/selective-reload-driver.ts) — asserts the full swap contract from the child process transcript:
  • fresh module instance per ?gen=N (captured=reloaded proves a new evaluation)
  • old plugin's fiber disposed only after the new one applied; its timers never fire again
  • root-owned effects tick through the swap unduplicated, disposed exactly once at shutdown
• tests/bun/repros/selective-reload.ts — manual narrative version (README updated)
• Docs — Phase C section in BUN_COMPATIBILITY.md rewritten from "deferred" to "primitive done" (remaining production gaps honestly listed: fs-watch → generations, config preservation, rollback); Phase 10 recorded in BUN_PORT_STATUS.md

Capability gating (not version gating)

The spec probes the runner binary for query-busting behavior and skips cleanly when absent — so:

• CI today: skip (stock Bun 1.3.14 lacks the fix) — no red, no green lie
• The moment a shipped Bun includes #39426, the test activates automatically
• Same gating pattern as the existing hot.spec.ts (#32856-gated)

Test plan

• bun test tests/bun/selective-reload.spec.ts against a release build of the PR branch (1.4.0-canary.1+c16333e9e) — 1 pass / 14 expects (transcript-ordering assertions); also verified against the debug build
• Same spec on stock Bun (no capable binary) — clean skip
• Full suite with a capable build present — 69/69 (68 + this test)
• Real-app smoke on the release binary: bin.bun.js boots Cordis, loads a plugin, SIGINT disposes the root, exit 0
• Runner detection: BUN_QUERY_BUSTING_BIN (now resolve()d — the driver child runs from a temp cwd) → .upstream/bun/build/release/bun → build/debug/bun-debug → skip
• Full Cordis suite runs green on the bun#39426 build overall (drop-in compatibility verified)

Notes

• Stacked on feat/bun-compat (feat: first-class Bun compatibility (core, timer, logger-console, loader, include) #1); merge feat: first-class Bun compatibility (core, timer, logger-console, loader, include) #1 first, then this (or rebase freely — one commit).
• Depends on an unmerged upstream PR. If fix(loader): keep a file:// URL's query in module keys oven-sh/bun#39426 is rejected, the spec stays skipped forever and this remains documentation of intent; if it lands, the gating can be dropped when CI's Bun is re-pinned.