#214 Port importBundleByPath ENOENT fallback to datastore/driver/vault/report loaders

Summary

Follow-up to swamp-club#212 / PR systeminit/swamp#1288. The fix in #1288 added two layers of defense against missing cached bundle files:

1. A bundle-existence gate in findStaleFiles (domain-shared — all six loaders inherit it via bundle_freshness.ts).
2. An ENOENT fallback in UserModelLoader.importBundleByPath that rebundles from source on demand.

Layer 1 covers all six loaders. Layer 2 (the import-site fallback) was only added to UserModelLoader. The four sibling loaders — UserDatastoreLoader, UserDriverLoader, UserVaultLoader, UserReportLoader — each have their own importBundle / loadSingleType paths that read entry.bundle_path directly with no ENOENT recovery. They benefit from layer 1 transparently, but they still lack the import-site defense for two cases:

• Concurrent-rm races: a swamp data gc or external rm between findStaleFiles and the loader's import call can ENOENT.
• Hot-load attach paths analogous to UserModelLoader.attachPendingExtensionsForType that may bypass freshness in some flows.

Reproducer

Same pattern as swamp-club#212, but targeting a sibling kind:

1. Define a custom datastore (or driver/vault/report) under the appropriate extensions/ path.
2. Run any command that loads it so the catalog and bundle exist.
3. While that command is still resolving the type, rm the bundle file out from under it (or rely on swamp data gc to misfire).
4. The catch path will throw NotFound from Deno.readTextFile(entry.bundle_path) instead of recovering.

The buildIndex path is fully covered by the layer-1 gate, so this is the residual surface only — narrow but a real hole for racy operations.

Suggested fix

Apply the same surgical pattern to each sibling loader's importBundle-equivalent:

• Catch Deno.errors.NotFound from Deno.readTextFile(entry.bundle_path).
• Rebundle from source via the loader's existing bundleExtension call (each loader already has one for first-load and stale-rebuild paths).
• Write the rebundled JS back to entry.bundle_path and continue with the import.
• Document the single-import-per-process invariant (per #1288's comment).

Each sibling loader has its own recoverMissingBundle shape candidate. Could also consider hoisting the helper into a shared utility — but the loaders' bundle paths and helpers diverge enough that copy-paste is probably cleaner than premature abstraction.

Environment

• swamp main as of v20260502.153639.0-sha.907c6883 (the merge that landed #1288)
• Affects all four sibling loaders symmetrically — Linux, macOS, Windows