feat: implement TFTP protocol crate (pfs-tftp-proto)

This commit introduces a sans-IO TFTP protocol implementation following
RFC 1350. The protocol crate provides:

- Packet types: RRQ, WRQ, DATA, ACK, ERROR with full serialization/parsing
- Error codes as defined in RFC 1350 Appendix
- Transfer modes: octet (binary) and netascii
- Client and server state machines for managing protocol flow
- Comprehensive tests for all packet types and state transitions

The sans-IO design separates protocol logic from I/O operations, making
the code testable and reusable across different I/O implementations.

Key design decisions:
- Mail mode is explicitly rejected as obsolete per RFC 1350
- Block numbers wrap around at 65535 for large file support
- State machines emit events that tell the I/O layer what to do next
- All protocol-specific values are documented with RFC citations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

crates/pfs-tftp-proto/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "pfs-tftp-proto"
+description = "Sans-IO TFTP protocol implementation (RFC 1350)"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+
+[lints]
+workspace = true
+
+[dependencies]
+thiserror = "2"
+
+[dev-dependencies]