feat(pdftract-29z7b): implement unified diagnostic system + CLI commands
- Added `cmd_explain_diagnostic` function to CLI for detailed diagnostic code explanation
- Added `--list-diagnostics` and `--explain-diagnostic <code>` CLI commands
- Verified all Phase 1.1-1.5 modules use unified DiagCode (lexer, parser, xref, stream, catalog, outline, pages)
- DIAGNOSTIC_CATALOG provides metadata for all 61 diagnostic codes
- Diagnostic struct size: 56 bytes (within 48-64 target range)
- emit! macro provides ergonomic diagnostic emission
- INV-8 maintained: no panics in error paths
All diagnostic codes follow naming convention:
- STRUCT_*: PDF structure errors
- STREAM_*: Stream decoder errors
- XREF_*: Cross-reference table errors
- ENCRYPTION_*: Encryption-related errors
- OCR_*: OCR pipeline errors
- REMOTE_*: Remote source errors
- PAGE_*: Page-level errors
- FONT_*: Font pipeline errors
- GSTATE_*: Graphics state errors
- LAYOUT_*: Layout and reading order errors
- MCP_*: MCP server errors
- CACHE_*: Cache errors
References: Phase 1.6 (error recovery), INV-8, Phase 0.4 (clippy enforces doc comments)
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