docs: merge tooling findings into SYNTHESIS/GOTCHAS/CORPUS_* and add handoff

Patches the top-level corpus docs with the 13 findings flagged during the
2026-04-18 canonical tooling research pass. tooling/README.md now marks each
finding [merged: <file>] or [flagged] for provenance.

- CORPUS_ollama_variants.md: annotate gemma4:26b as MoE (25.2B total / 3.8B
  active, 8-of-128 experts + 1 shared). Note Q4_K_M inference is standard
  (the "MoE quality degrades at 4-bit" caveat is training-only). Add note
  that audio on E-series is NOT available via Ollama — llama.cpp mmproj
  or vLLM only.
- CORPUS_capabilities.md: native system role, configurable thinking mode,
  first trained tool use (vs Gemma 1/2/3 proof-of-concept), native object
  detection with bbox output in 1000x1000 coords, pointer to EmbeddingGemma
  for retrieval (Gemma 4 has no embedding mode).
- CORPUS_tool_calling_format.md: add Chat Template Context section
  documenting the <|turn>/<turn|> asymmetric brackets (new in Gemma 4,
  replaced <start_of_turn>/<end_of_turn>) plus <|think>, <|channel>,
  <|image>, <|audio> tokens. Add HF transformers Alternative section
  showing processor.parse_response with response_schema.
- GOTCHAS.md: add MEDIUM gotcha for abandoned google/gemma_pytorch (no
  Gemma 4 support since 2025-05-30). Expand fine-tuning section with FA2/FA4
  head_dim=512 break, fused LoRA kernel issues, 26B A4B training-quant
  guidance, new tool-call tokens as learned embeddings.
- SYNTHESIS.md: add banner pointing to tooling/ for canonical upstream
  material. Add embeddinggemma row to Model Selection table.

Also:
- Add .gitignore excluding .backup/ (local scratch per global CLAUDE.md
  convention, not needed in tracked history) and __pycache__/.
- Add .claude/handoffs/2026-04-18-canonical-tooling-research.md so future
  sessions can pick up cold — facts verified, open threads, what changed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,12 @@
+# Local scratch / backups — per ~/.claude/CLAUDE.md, Claude keeps backups before
+# editing any file. Useful locally; not useful in the tracked history.
+.backup/
+
+# Python
+__pycache__/
+*.pyc
+.pytest_cache/
+
+# Editor / OS
+.DS_Store
+*.swp