test(proto): add codec property tests

Add proptest-based property tests for irtt-proto's varint and Params
codecs, covering the full u64/i64 domains for varint/zigzag round trips
and generated valid Params for semantic round trips, presence
omission rules, repeated-tag last-wins behavior, and unknown-tag
tolerance.

crates/irtt-proto/Cargo.toml

@@ -16,3 +16,6 @@ hmac = "0.13"
 md-5 = "0.11"
 subtle = "2.6.1"
 thiserror = "2.0.20"
+
+[dev-dependencies]
+proptest = "1.11.0"