docs(workflow): add FP-series fork platform change convention (ANGA-555)

[anhermon]

Summary

• Adds doc/F-SERIES-CONVENTION.md defining the FP<N> identifier scheme for all fork platform-level changes (OS-specific fixes, stale-lock patches, adapter compat)
• Adds .github/ISSUE_TEMPLATE/platform-change.md requiring the Fork Platform Flag block on every platform issue
• Backfills FP1–FP5 registry entries for the existing fix chain (stale-lock rounds FP1/FP2/FP3, Windows compat FP4/FP5-PARTIAL)

Closes ANGA-555. Part of ANGA-809 PR discipline improvements.

Fork Platform Flag

Field	Value
Flag ID	FP6 (this PR establishes the convention itself)
Scope	all
Status	applied
Verification	doc/F-SERIES-CONVENTION.md exists and registry has FP1–FP5 entries
Known Gaps	none — future platform changes will add rows to the registry
Upstream Alignment	none
Related Issues	ANGA-555

Upstream Conflict Check

• Searched upstream tracker (paperclipai/paperclip) — upstream uses bare FN namespace; fork uses FPN to avoid collision
• Prior Art Link: none
• Reconciliation Scope: deliberate fork divergence (separate namespace)

Test plan

• doc/F-SERIES-CONVENTION.md renders correctly on GitHub
• .github/ISSUE_TEMPLATE/platform-change.md appears in GitHub's "New issue" template picker
• Registry FP1–FP5 entries all link to valid ANGA issues
• Backfill comments posted on ANGA-262, ANGA-263, ANGA-295, ANGA-147

🤖 Generated with Claude Code

[anhermon]

@anhermon — PR #82 is ready to merge. All 5 CI checks passed, and User Agent gave LGTM on Paperclip (ANGA-555). Branch protection requires owner approval. Please approve and merge when convenient.