refactor(dpi/quic): single-allocation connection_id_to_hex

[0xghost42]

Summary

connection_id_to_hex was rendering a QUIC connection ID through the worst-case allocation pattern: format!("{:02x}", b) allocated a 2-byte String for every input byte, the iterator collected them into a Vec<String> (one more allocation), and join(":") then walked the vector to build the final String. For an 8-byte short-form DCID that is 8 + 1 + 1 = 10 heap allocations per render.

Replace with a single String::with_capacity(id.len() * 3 - 1) (exact final size: 2 hex chars per byte + N-1 colon separators) and a per-byte write!. Writing into a pre-sized String does not reallocate, so the helper now performs exactly one heap allocation per call regardless of input length, except for the empty-slice case which returns an empty String (no allocation). The lowercase, colon-separated output is unchanged.

Tests

Four regression tests added:

• An 8-byte representative DCID — locks the lowercase + colon-separated contract.
• A mix of single-digit bytes (0x00..=0x0f) — locks the zero-padding contract that {:02x} provides.
• A single-byte input — locks that no trailing separator is emitted.
• The empty-slice base case.

Local checks:

• cargo test --lib — 365 passed.
• cargo clippy --all-targets -- -D warnings — clean.
• cargo fmt --check — clean.

Closes #305

[laundmo]

This PR, lik the other, is lacking any proof that this is a performance gain and or that the compiler does not optimize the current code to a single string write.