fix(import): resolve passthrough-stored assets referenced by absolute path

[tommy230]

Summary

An import bundle can store a binary under passthrough/wp-content/... while its HTML references /wp-content/.... resolveAndRecord strips the leading slash and does one exact FileMap lookup, so the asset is never planned. The unreferenced-media sweep still uploads the bytes, but nothing records a usage, and the published page 404s on the URL the HTML declares.

A WordPress export surfaced this: the site published cleanly except for its mobile logo and an address icon, both stored under passthrough/ and both referenced root-relative from every page.

After an exact miss, resolveAndRecord now retries the same key under passthrough/ before giving up. An exact key still wins when both exist, and external URLs and unresolvable paths behave as before.

Related: #349 reports unresolved references and matches punctuation drift, but its normalisation keeps path segments, so it cannot match the passthrough/ storage prefix. The two are complementary; this lookup slots into that PR's resolver chain if it lands first.

Verification

• bun run build
• bun test
• bun run lint
• Docker/deployment check, if relevant

New coverage in assetPlan.test.ts: a root-relative img src resolves to its passthrough-stored file, and an exact FileMap key is preferred over its passthrough counterpart. The first fails without the fix.

Checklist

• Tests cover behavior changes.
• Docs were updated when behavior, config, deployment, or public surfaces changed.
• No compatibility shim was added for old pre-release behavior.
• No secrets, local databases, uploads, or generated artifacts are included.