cpp-safety: safe-C++ playbook, fix a second UB, and a sanitizer net

Response to incident WA-2026-10-27 (the std::vector max_size() crash). Deep
research archived under research/safe-cpp-practices (8 Tier 1, 37 Tier 2, 2
conclusions) on production-safe C++ that keeps development speed: OpenSSF
compiler hardening, the C++ Core Guidelines bounds profile, integer-overflow
practice, libFuzzer/sanitizer CI, the sanitizer overhead ladder, clang-tidy
ratchets.

The research prescribed an ASan+UBSan+_GLIBCXX_ASSERTIONS build of the parsers.
Standing it up immediately caught a second, independent undefined-behavior bug
that two manual reviews had missed: a protobuf varint loop that shifted by up
to 70 bits (shift >= 64 is UB) on malformed input. Fixed and guarded by a
deterministic test and the sanitizer net.

- src/whatsapp/wa_proto.cpp: guard the wire-0 varint shift (the UB).
- tests/whatsapp_test.cpp: add an overlong-varint hostile case.
- tools/san-whatsapp.sh: the standing sanitizer net for detz::wa parsers.
- WaClient frame capture: a frame that fails to decode/dispatch has its exact
  bytes recorded to <home>/whatsapp/last-error.log (bounded), so the still
  unreproduced live <iq> failure becomes a one-paste deterministic fix. Wired
  from the wizard and the whatsapp_send tool.
- docs/cpp-safety.md: the project-wide playbook and design decisions (four
  layers: overflow-safe checked-reader parsing, cheap release hardening, the
  sanitizer test net, capture). docs/whatsapp.md section 13.6 records the
  follow-up and the honest open item. AGENTS.md makes it a standing rule;
  cap.md and mcp.md inherit it for their wire parsers.

Verified: build, whatsapp_test (incl. hostile+fuzz), make lint, and
tools/san-whatsapp.sh all green; research audit PASS.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

research/safe-cpp-practices/github-imshark-fuzz.md

@@ -0,0 +1,28 @@
+# imshark PR#13: add libFuzzer harnesses and fix parser bugs
+
+## Metadata
+
+| Field | Value |
+|---|---|
+| Institute | Molodetz Institute — Research & Retrieval Division |
+| Subject | safe-cpp-practices |
+| Document type | Source Citation Card |
+| Tier | 2 |
+| Source title | imshark PR#13: add libFuzzer harnesses and fix parser bugs |
+| Source URL | https://github.com/Zaryob/imshark/pull/13 |
+| Source type | repository |
+| Date researched | 2026-10-27 |
+| Researched by | tai (agent), model: Claude Opus 4.8, on behalf of retoor |
+| Research method | Search aggregator result listing (not independently opened or quoted) |
+| Confidence | Not independently verified |
+| Status | Active |
+
+## Why it's here
+
+Worked example of adding fuzzing and fixing the bugs it found in a parser.
+
+## Changelog
+
+| Date | Change |
+|---|---|
+| 2026-10-27 | Logged as consulted-but-not-analyzed. |