feat: const fn encode, decode optimization, comprehensive docs and examples

- Make `encode` const fn for compile-time COBS encoding (MSRV bumped to 1.83)
- Optimize decode by removing unnecessary branch in inner loop
- Add comprehensive rustdoc with examples for every function
- Add crate-level doc sections: quick start, buffer sizing, framing,
  stream parsing, compile-time encoding, error handling
- Expand README with decision matrix, examples section, testing depth
- Bump version to 0.2.0

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -1,8 +1,8 @@
 [package]
 name = "ucobs"
-version = "0.1.0"
+version = "0.2.0"
 edition = "2021"
-rust-version = "1.56"
+rust-version = "1.83"
 license = "MIT"
 description = "μCOBS — Consistent Overhead Byte Stuffing. no_std, zero-alloc, fastest, most-tested COBS implementation."
 authors = ["Stephen Waits <steve@waits.net>"]