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>

src/loss_report.py

@@ -0,0 +1,85 @@
+"""Per-export-run tally for content that was dropped or partially extracted.
+
+Surfaces the loss visibility that the rest of the system promises in its
+output (visible ``unknown`` blocks). The summary emitted at the end of
+each export is the load-bearing operator-facing signal: if a real content
+type starts being silently dropped, this is where it shows up.
+
+Pass a single instance through ``BaseProvider.normalize_conversation`` and
+read it back in ``src/main.py`` after the export loop. No global state.
+"""
+
+from collections import Counter
+from dataclasses import dataclass, field
+
+_TOP_N_BREAKDOWN = 5
+
+
+@dataclass
+class LossReport:
+    """Counters for things that didn't render cleanly in an export run."""
+
+    # Type-keyed counters. Values are int counts.
+    unknown_blocks: Counter = field(default_factory=Counter)
+    extraction_failures: Counter = field(default_factory=Counter)
+    filtered_roles: Counter = field(default_factory=Counter)
+
+    # Aggregate counters
+    messages_rendered: int = 0
+    conversations: int = 0
+
+    # Recording -------------------------------------------------------------
+
+    def record_unknown(self, raw_type: str) -> None:
+        self.unknown_blocks[raw_type or "?"] += 1
+
+    def record_extraction_failure(self, raw_type: str) -> None:
+        self.extraction_failures[raw_type or "?"] += 1
+
+    def record_filtered_role(self, role: str) -> None:
+        self.filtered_roles[role or "?"] += 1
+
+    def record_message(self) -> None:
+        self.messages_rendered += 1
+
+    def record_conversation(self) -> None:
+        self.conversations += 1
+
+    # Summary ---------------------------------------------------------------
+
+    def format_summary(self) -> str:
+        """Return a multi-line summary table suitable for INFO logging.
+
+        Format pinned by plan §Post-export summary — "(none)" sentinel when a
+        counter is empty, top-5 breakdown with "+ N more types" overflow.
+        """
+        lines: list[str] = ["[export] Run summary:"]
+        lines.append(f"  conversations:        {self.conversations}")
+        lines.append(f"  messages rendered:    {self.messages_rendered}")
+        lines.extend(_format_section("unknown blocks:      ", self.unknown_blocks))
+        lines.extend(_format_section("extraction failures: ", self.extraction_failures))
+        lines.append(
+            "  filtered roles:       "
+            "(filter lifted in v0.4.0 — counter retained for future use, expected 0)"
+        )
+        if self.filtered_roles:
+            for role, count in self.filtered_roles.most_common(_TOP_N_BREAKDOWN):
+                lines.append(f"                        {role}={count}")
+        return "\n".join(lines)
+
+
+def _format_section(label: str, counter: Counter) -> list[str]:
+    """Render one counter section: header line + indented breakdown lines."""
+    total = sum(counter.values())
+    header = f"  {label} {total}"
+    if total == 0:
+        return [header, "                        (none)"]
+
+    lines = [header]
+    most_common = counter.most_common()
+    for raw_type, count in most_common[:_TOP_N_BREAKDOWN]:
+        lines.append(f"                        {raw_type}={count}")
+    if len(most_common) > _TOP_N_BREAKDOWN:
+        remainder = len(most_common) - _TOP_N_BREAKDOWN
+        lines.append(f"                        + {remainder} more types")
+    return lines