feat: v0.4.0 — rich content support with typed blocks and loss visibility

Extracts per-message content into a typed `blocks` list (text, code,
thinking, tool_use, tool_result, image_placeholder, file_placeholder,
unknown) and renders them at exporter write time. Voice transcripts,
Custom Instructions, and image references now appear in exports
instead of being silently dropped.

Foundation:
- src/blocks.py: pure block constructors, _safe_fence (fence-corruption
  defense, verified live in Joplin), _blockquote_prefix, render
- src/loss_report.py: per-run tally surfaced as INFO summary at end of
  export so silently-dropped data becomes visible

Providers:
- ChatGPT: dispatch on content_type produces typed blocks; voice shapes
  (audio_transcription, audio_asset_pointer, real_time_user_audio_video_
  asset_pointer) locked from live DevTools capture; Custom Instructions
  bug fix (parts-vs-direct-fields); role filter lifted; hidden-context
  marker driven by is_visually_hidden_from_conversation flag
- Claude: defensive dispatch for text/thinking/tool_use/tool_result/image
  with recursive nested-block flattening; untested against real rich-
  content data — fix-forward in v0.4.1

Exporter:
- Markdown renders from blocks at write time via render_blocks_to_markdown;
  backward-compat fallback to content for any pre-v0.4.0 cached data

Tests:
- 27 new tests across providers, exporters, CLI; fixtures rebuilt with
  real-shape ChatGPT voice + Custom Instructions cases
- 181/181 pass

Behavior changes (intentional):
- JSON output omits content; consumers should read blocks
- Per-conversation message counts increase (Custom Instructions, image-
  only, tool-only messages now appear)
- Existing exports not auto-re-rendered; users wanting fresh output run
  cache --clear then export

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/fixtures/claude_conversation.json

@@ -30,6 +30,15 @@
       "sender": "human",
       "created_at": "2024-06-10T14:45:00.000Z",
       "content": "Thank you, that helped!"
+    },
+    {
+      "uuid": "msg-004",
+      "sender": "human",
+      "created_at": "2024-06-10T14:50:00.000Z",
+      "content": [
+        {"type": "text", "text": "What about this image?"},
+        {"type": "image", "source": {"file_uuid": "claude-image-uuid-1", "media_type": "image/png"}}
+      ]
     }
   ]
 }