processor: refactor to two-phase commit for inbox processing

Introduce postPlan to capture everything validated in Phase 1 before
any filesystem mutation occurs. Phase 1 scans the inbox, validates all
source files (parses text, markdown, image, audio), checks markdown
image claims for conflicts, and collects a plan per item. Phase 2
commits mutations only after every plan is validated: creates post
directories, writes assets, persists post.json, and removes sources.

If Phase 1 fails (e.g. unsupported file, missing markdown image, claim
conflict), no mutations occur and the inbox is left untouched. Roll
back the partial post directory if a mutation fails during commit.

Also refactor image and audio sub-processors into validation-only and
write-only parts (validateImage/writeImageAsset, validateAudio/copyFile)
so that Phase 1 is strictly read-only.

All existing tests pass.

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