docs(pdftract-2cnmr): add verification note for PdfSource trait implementation

notes/pdftract-2cnmr.md

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+# pdftract-2cnmr: PdfSource trait + MmapSource + FileSource
+
+## Summary
+
+The PdfSource trait abstraction with MmapSource and FileSource implementations was already implemented in the codebase prior to this bead. All core acceptance criteria are met.
+
+## Implementation Status
+
+### PASS Criteria
+
+1. **PdfSource trait defined and exported** ✓
+   - Location: `crates/pdftract-core/src/source/mod.rs`
+   - Provides `len()`, `read_range(offset, length)`, and `prefetch()` methods
+   - Object-safe trait with `Read + Seek + Send + Sync` bounds for rayon parallelism
+
+2. **MmapSource implementation** ✓
+   - Location: `crates/pdftract-core/src/source/mmap.rs`
+   - Uses `memmap2 = "0.9"` for memory-mapped I/O
+   - Implements `advise_sequential()` for MADV_SEQUENTIAL hint
+   - Comprehensive test suite (29 tests) covering all operations
+
+3. **FileSource implementation** ✓
+   - Location: `crates/pdftract-core/src/source/file_source.rs`
+   - Uses `parking_lot::Mutex` for thread-safe `&self` access
+   - Handles special files (e.g., /proc) that don't support mmap
+   - Comprehensive test suite (15 tests)
+
+4. **MemorySource implementation** (bonus)
+   - Location: `crates/pdftract-core/src/source/memory.rs`
+   - In-memory source for testing with zero-copy Bytes
+
+5. **Exports from lib.rs** ✓
+   - All source types re-exported from `pdftract_core::source`
+   - Line 90: `pub use source::{FileSource, MmapSource, PdfSource};`
+
+### Dependencies
+
+All required dependencies are present in `Cargo.toml`:
+- `bytes = "1"` - Zero-copy slice type
+- `memmap2 = "0.9"` - Memory mapping
+- `parking_lot = "0.12"` - Mutex for FileSource
+
+### Build Status
+
+- **Lib compilation**: PASS - `cargo build --package pdftract-core --lib` succeeds
+- **Test compilation**: BLOCKED - Unrelated test errors in `markdown.rs` (API signature mismatches in test-only code)
+
+The source module itself compiles cleanly. Test compilation errors are in test code for the markdown module (`block_to_markdown` and `page_to_markdown` function calls with outdated signatures), which is unrelated to the source abstraction work.
+
+## Code Quality
+
+- All implementations include comprehensive test suites
+- Thread-safe (`Send + Sync`) for rayon parallelism
+- Proper error handling with `io::Result`
+- Well-documented with examples and safety notes
+
+## INV-8 Verification
+
+INV-8 (Invariants) appears to be maintained:
+- Source abstraction properly abstracts over storage medium
+- Thread safety enforced via trait bounds
+- No assumptions about file mutability (documented as known limitation for mmap)
+
+## Files Examined
+
+1. `crates/pdftract-core/src/source/mod.rs` - PdfSource trait definition
+2. `crates/pdftract-core/src/source/mmap.rs` - MmapSource implementation
+3. `crates/pdftract-core/src/source/file_source.rs` - FileSource implementation
+4. `crates/pdftract-core/src/source/memory.rs` - MemorySource implementation
+5. `crates/pdftract-core/src/lib.rs` - Public exports
+6. `crates/pdftract-core/Cargo.toml` - Dependencies verified
+
+## Conclusion
+
+The PdfSource trait and implementations are complete and meet all acceptance criteria. The bead work was already done in a prior commit. No new code changes were required for this bead.