Trace: extract and replace pasted images in Pi transcripts #3716

Problem

The compatibility matrix marks Pi pasted-image pointers as No. User-pasted image data remains inline in the Pi transcript instead of being stored once as a spec material and replaced with a pointer.

Scope

Implement Pi's trace.PastedImageFinder and extend its spec-copy replacement path for the native image block shapes that Pi persists.

Acceptance criteria

• Extract valid user-pasted image data from supported Pi message blocks.
• Deduplicate decoded bytes and assign deterministic Pasted image N ordering.
• Exclude tool-result images and non-paste image records.
• Replace inline copies only after the material is stored successfully.
• Keep invalid data, remote URLs, path-only records, and storage failures inline without blocking push.
• Reuse an already captured identical image instead of creating a duplicate material.
• Add finder, replacer, action, and malformed-input regressions.
• Run the repeated-image live scenario and verify one stored material and no supported inline duplicate.
• Update the compatibility matrix from measured results.

Non-goals

• Treating tool-result images as user pastes.
• Fetching remote image URLs.

Dependencies

• Pi spec-source pointer tracker, because this extends the same provider replacement path.
• Design reference: Trace: replace pasted images in the session transcript with spec pointers #3569.
• Baseline Pi support: feat(trace): add Pi provider support #3704.