plan(sprint-13/preflight): PP-9 PR_ARC backfill — 8 sprint-11/12 PR entries (#383..#390)

PP-9 (Opus) prepended 8 PR entries to PR_ARC_INVENTORY.md (1537→1903
lines, +366 / +50,619 bytes). APPEND-ONLY rule respected — no prior
entries touched. Top→bottom reverse chronological:

#390 sprint-12/wave-G (In PR, `3324b64`) — grade A−. D-CSV-5b cutover
   + D-CSV-6b WitnessCorpus + D-CSV-13 batch + D-CSV-15 Jirak math.
#389 sprint-12 wave-F codex P2 follow-up — AttentionMaskBackend impl
   + canonical MailboxId (CSI-10).
#388 sprint-12 Wave F fleet (12 Sonnet + 1 Opus) — grade B. D-CSV-
   10/11/12 scaffolds + AttentionMask + plan v2.
#387 sprint-11 Wave E — grade A−. D-CSV-8 MUL i4 scalar + D-CSV-9
   8-channel transcoder (Option R-3).
#386 sprint-11 Wave D — grade B+. D-CSV-7 MailboxSoA + D-CSV-6a
   WitnessCorpus core.
#385 sprint-11 Wave C — grade B+. D-CSV-5a sibling QualiaI4Column.
#384 sprint-11 Wave B — grade A. D-CSV-2 QualiaI4_16D + OQ-CSV-1
   Option α ratification.
#383 sprint-11 Wave A — grade A−. D-CSV-1 v2 layout + D-CSV-3 signed
   mantissa + D-CSV-4 CollapseGateEmission.

Every entry mirrors the #381/#379 template — Header / Confidence /
Added / Locked / Deferred / Docs / Cross-refs — with forward+backward
linkages (#383→#385→#390 D-CSV-5 chain; E-META-10 catch in #383 →
iron-rule promotion in #390 W-G5; etc.).

Total PR headers in file: 28 (was 20). 13 of 14 planners done.
Only PP-13 (brutally-honest-tester agent) still in flight.

https://claude.ai/code/session_01UwJuKqP828qyX1VkLgGJFS

Author: claude