mt038

mt038: harden multimodal flood path (A through F.2)

Coordinated fix bundle that took 20-customer flood-test success
from 17/20 (median 165s) to 20/20 (median 33s) across the new
image+card mix:

  A: re-scrape rescue on stale_reply_source_msg_id
  B: defer dispatch when scrape fails on opaque attachment marker
  C: source-guard recognizes .chatd-card product-card bubbles
  D: placeholder sweeper survives CDP recovery (task-state gate)
  E: placeholder cancel/suppress handles arm/cancel key-mismatch
  F.2: mt030 honors mt017's pre-existing baseline tag

Plus mt019/20 emulator gap: RAG fault injection (hang/error mode)
in agent/ec_skills/rag/local_rag_mcp.py, mirroring the existing
LLM 429 fault path.

Code commit:  6fd278254
Docs commit:  c1079c0f2
Tests: tests/test_mt038_attachment_marker_rescue.py (44 cases)
Full regression: mt030/31/33/34/36/37/38 — 123 pass.