refactor(core): replace cheerio with linkedom to drop deprecated whatwg-encoding (#187)

## Summary

- `cheerio` pulls `encoding-sniffer` → `whatwg-encoding@3.1.1` (deprecated), causing a warning on every `npm install -g hyperframes`
- `linkedom` was already bundled into the CLI via tsup `noExternal` and has zero deprecated transitive deps
- Rewrote `htmlBundler.ts` and `subComposition.ts` to use standard DOM APIs via `linkedom`
- Added a `parseHTMLContent` helper that wraps HTML fragments in a full document structure (required for `linkedom` to populate `document.body`)
- Removed `cheerio` from `cli` dependencies and tsup `external` list
- Replaced `cheerio` with `linkedom` in `core` `optionalDependencies`

## Test plan

- [x] All 411 tests pass (`bun run test` in `packages/core`)
- [x] Full monorepo build succeeds (`bun run build`)
- [x] TypeScript typecheck passes

packages/core/package.json

@@ -109,7 +109,7 @@
     }
   },
   "optionalDependencies": {
-    "cheerio": "^1.2.0",
-    "esbuild": "^0.25.12"
+    "esbuild": "^0.25.12",
+    "linkedom": "^0.18.12"
   }
 }