Trace: replace exact Pi spec-source copies with pointers #3715

Problem

The compatibility matrix marks Pi spec-source pointers as No. Exact copies of captured text specifications remain inline in Pi raw-session evidence, duplicating material content.

Scope

Implement Pi's trace.SpecCopyReplacer using the existing pointer pipeline. Replace only exact supported reads and writes of captured text sources while preserving transformed, partial, or ambiguous content.

Acceptance criteria

• Implement SpecCopyReplacer for Pi with a compile-time interface check.
• Replace exact full-file reads and writes with pointer.New(digest) across every Pi raw-session stream.
• Keep partial, transformed, computed, and unsupported copies inline.
• Return correct replaced/skipped finder reports.
• Cover selected root and supported child streams once Pi subagent evidence is available.
• Add malformed-content and ordering regressions.
• Run the live spec-pointer scenario and confirm the stored material resolves while supported duplicates are absent.
• Update the compatibility matrix from measured results.

Non-goals

• Image extraction or pasted-image deduplication.
• Rewriting arbitrary shell output that cannot be matched exactly.

Dependencies

• Pi subagent evidence tracker, for child-stream coverage.
• Design references: AI coding session evidence: replace inline base64 images with placeholders #3556 and Trace: replace pasted images in the session transcript with spec pointers #3569.
• Baseline Pi support: feat(trace): add Pi provider support #3704.