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>

Cargo.toml

@@ -1,14 +1,17 @@
-[package]
-name = "pfs-tftp"
+[workspace]
+resolver = "3"
+members = ["crates/*"]
+
+[workspace.package]
 version = "0.1.0"
 edition = "2024"
+license = "MIT"
+repository = "https://github.com/pfs/pfs-tftp"
 
-[lints.rust]
+[workspace.lints.rust]
 unsafe_code = "forbid"
 
-[lints.clippy]
+[workspace.lints.clippy]
 pedantic = { level = "warn", priority = -1 }
 todo = "warn"
 unwrap_used = "warn"
-
-[dependencies]