docs: record Linear sync projection gaps

[ConnorBritain]

Summary

• records four Linear sync gaps found during a real five-PI / 38-slice projection
• separates observed behavior, operational impact, and recommended acceptance criteria
• keeps this PR documentation-only; no sync behavior changes under the submission deadline

Validation

• reproduced against the Pidgeon Health roadmap on 2026-07-13
• git diff --check

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