fix: correct FRAME_HEADER_SIZE (24→20) + add protocol frame codec recovery tests

[dev-khimesh]

Summary

Fix pre-existing frame header size bug and add 9 deterministic regression tests for protocol frame codec recovery.

Bug Fix

FRAME_HEADER_SIZE was incorrectly set to 24 but the actual wire format header is only 20 bytes (magic:4 + version:1 + type:1 + flags:2 + payload_len:4 + sequence:4 + reserved:4). This mismatch caused ALL existing codec tests to fail because the decoder waited for 4 extra bytes that never arrived.

Tests Added (13 total, all pass)

• Recovery after truncated input: partial header/payload → Ok(None) without panicking
• Invalid frame length: payload beyond MAX → Err(MessageTooLarge)
• Invalid magic/version: wrong magic → Err(InvalidMessage), bad version → Err(UnsupportedVersion)
• State preservation: failed decode does not corrupt decoder state
• Recovery: valid frame after invalid input decodes successfully
• Bulk decoding: decode_all() collects all frames
• Reset: reset() preserves future decode capability

Validation

cargo test --lib protocol::codec  # 13/13 pass

Closes #2