docs(bf-4p23w): verify cargo fuzz toolchain installation

notes/bf-4p23w.md

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+# Verification: cargo fuzz toolchain installation
+
+**Date:** 2026-07-05
+**Bead:** bf-4p23w
+**Status:** PASS
+
+## Acceptance Criteria
+
+### 1. `cargo fuzz --version` runs without error
+✅ **PASS** - Command succeeds and reports version 0.13.1
+
+```bash
+$ cargo fuzz --version
+cargo-fuzz 0.13.1
+```
+
+### 2. Fuzz toolchain is present in `rustup show`
+✅ **PASS** - Nightly toolchain is installed and active
+
+```bash
+$ rustup show
+Default host: x86_64-unknown-linux-gnu
+installed toolchains
+--------------------
+stable-x86_64-unknown-linux-gnu (default)
+nightly-x86_64-unknown-linux-gnu (active)
+...
+```
+
+The active toolchain in the pdftract workspace is `nightly-x86_64-unknown-linux-gnu`, which is required for cargo-fuzz.
+
+### 3. No error messages about missing fuzz toolchain
+✅ **PASS** - All commands execute successfully with no errors
+
+## Additional Verification
+
+- ✅ `cargo fuzz --help` works correctly, showing all available commands (init, add, build, run, cmin, tmin, coverage, etc.)
+- ✅ `cargo fuzz list` successfully lists 6 existing fuzz targets:
+  - cmap_parser
+  - content
+  - lexer
+  - object_parser
+  - stream_decoder
+  - xref
+- ✅ Fuzz directory structure exists with proper layout (`fuzz/fuzz_targets/`, `fuzz/corpus/`, `fuzz/artifacts/`)
+
+## Conclusion
+
+The cargo fuzz toolchain is properly installed and fully functional. All acceptance criteria are met.