CS-11022: emit boxel-ui-checksum.txt from vite host build

[lukemelia]

Summary

Restores the cross-deploy boxel-ui-change signal that the webpack build produced. The webpack plugin (packages/host/lib/build/package-dist-checksums.js) was deleted in commit `0f24e3c06e` as part of the vite migration, but its consumer (`packages/realm-server/lib/boxel-ui-change-checker.ts` + `POST /_post-deployment`) was left in place — so on a vite-built host, `/boxel-ui-checksum.txt` 404s and the post-deploy reindex never fires when boxel-ui changes.

Approach

New vite/rollup plugin at `packages/host/lib/build/boxel-ui-checksum-plugin.js`. Same algorithm as the webpack plugin so an existing `/persistent/boxel-ui-checksum.txt` on EFS stays comparable across the cutover:

• Walk `packages/boxel-ui/addon/dist` recursively, skip `.map` files
• Sort files, hash `relPath:sha256(content)` stream
• Emit `boxel-ui-checksum.txt` at the bundle root via `this.emitFile`

Build-only (`apply: 'build'`) — dev server doesn't need a static checksum and walking the dist tree on every dev restart isn't worth the latency.

Verification

Smoke-tested the plugin in isolation (the local checkout predates the vite merge so a full `pnpm install + vite build` wasn't on hand):

• Direct `calculateBoxelUIChecksum()` returns a deterministic 64-char SHA256 across runs
• `generateBundle` emits one asset, `{ type: 'asset', fileName: 'boxel-ui-checksum.txt', source: }` — exactly what `compilation.emitAsset` produced before
• Missing-dist path warns and emits nothing (matches prior behavior)

CI's `vite build` step should produce the asset; reviewers can confirm by checking that `packages/host/dist/boxel-ui-checksum.txt` shows up in the build output artifact.

Test plan

• CI: host tests pass
• Manual (after deploy): `curl /boxel-ui-checksum.txt` returns a 64-char hex string
• Manual (after deploy): `POST /_post-deployment` against staging realm-server returns 200 (no longer throwing on the boxel-ui checksum fetch)

🤖 Generated with Claude Code

[github-actions]

Preview deployments

• Host staging preview

• Host production preview

Host Test Results

    1 files  ±    0      1 suites  ±0   3h 54m 6s ⏱️ + 2h 5m 48s
2 565 tests +  208  2 549 ✅ +  218  15 💤 ± 0  0 ❌ ± 0  1 🔥  - 10 
5 168 runs  +2 793  5 136 ✅ +2 798  30 💤 +15  1 ❌  - 10  1 🔥  - 10 

Results for commit f56976b. ± Comparison against earlier commit b51cc1c.

For more details on these errors, see this check.

Realm Server Test Results

    1 files  ±0      1 suites  ±0   16m 28s ⏱️ +40s
1 248 tests ±0  1 248 ✅ ±0  0 💤 ±0  0 ❌ ±0 
1 326 runs  ±0  1 326 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit f56976b. ± Comparison against earlier commit b51cc1c.

[Copilot]

Pull request overview

This PR restores the boxel-ui-checksum.txt artifact in Vite-built host bundles so realm-server’s post-deployment logic can detect cross-deploy Boxel UI changes and trigger a full reindex (matching the legacy webpack behavior that previously produced this file).

Changes:

• Add a Vite/Rollup build-only plugin that computes a deterministic SHA256 over packages/boxel-ui/addon/dist (excluding .map) and emits boxel-ui-checksum.txt at the bundle root.
• Wire the new plugin into packages/host/vite.config.mjs so it runs during vite build.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
packages/host/vite.config.mjs	Imports and registers the new checksum-emitting Vite plugin during build.
packages/host/lib/build/boxel-ui-checksum-plugin.js	Implements checksum calculation + Rollup generateBundle emission of boxel-ui-checksum.txt.

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