ieee80211: preserve all 12 bits of the A-MPDU delimiter length

[mgonzalezlopezudc]

What

Preserve all 12 bits of the A-MPDU delimiter length when deserializing Ieee80211MpduSubframeHeader in Ieee80211MpduSubframeHeaderSerializer.

Why

In Ieee80211MpduSubframeHeaderSerializer::deserializeFields(), the decoder previously discarded the high length nibble and overwrote the length with only the low byte:

stream.readUint4();
mpduSubframe->setLength(stream.readUint4() >> 8);
mpduSubframe->setLength(stream.readUint8());

Because readUint4() >> 8 evaluates to 0 and the subsequent setLength(stream.readUint8()) unconditionally overwrote the length with only the low 8 bits, any delimiter length >= 256 had its upper 4 bits discarded (e.g., decoding 0xABC as 0x0BC).

Restoring:

int length = stream.readUint4() << 8;
length |= stream.readUint8();
mpduSubframe->setLength(length);

restores symmetry with serializeFields(), which emits the 12-bit length across the high nibble and low byte per IEEE Std 802.11 A-MPDU delimiter specifications.

Reading Order

This PR consists of a single self-contained commit:

1. 97fdb7ba2e — ieee80211: preserve all 12 bits of the A-MPDU delimiter length
   • Single decision & rationale: Fixes 12-bit length deserialization in Ieee80211MpduSubframeHeaderSerializer::deserializeFields and adds unit test Ieee80211MpduSubframeHeaderSerializer_1.test verifying round-trip serialization/deserialization across byte boundaries (0, 255, 256, 0xABC, 4095) and fixed-byte decoding.
   • Component surface:
     • src/inet/linklayer/ieee80211/mac/Ieee80211MacHeaderSerializer.cc
     • tests/unit/Ieee80211MpduSubframeHeaderSerializer_1.test

Architectural Surface

• Modules touched:
  • src/inet/linklayer/ieee80211/mac/Ieee80211MacHeaderSerializer.cc
  • tests/unit/Ieee80211MpduSubframeHeaderSerializer_1.test
• Contracts and protocols: IEEE 802.11 MAC A-MPDU subframe header chunk serializer (Ieee80211MpduSubframeHeaderSerializer).
• Packet representation: Preserves existing Ieee80211MpduSubframeHeader chunk definition; fixes deserialization of byte-backed packets into chunk fields.
• Configuration surface: None.
• Feature descriptors: None.
• Seals and audit exceptions: All modified source files under src/inet/ are unsealed (doc/project/enforcement/check-source-seals.sh PASS). No architectural deviations (AV-*) or naming deviations (NV-*) introduced.

Baselines

No fingerprint or statistical baseline updates required.

Verification Evidence

1. Compilation:
   • make MODE=debug -j$(nproc): Clean build (libINET_dbg.so).
   • make MODE=release -j$(nproc): Clean build (libINET.so).
2. Mechanical gates:
   • doc/project/enforcement/check-commits.sh upstream/master..HEAD: PASS (1 commit, linear, valid subject/facts format, clean splits).
   • doc/project/enforcement/check-source-seals.sh --base upstream/master: PASS (all touched files unsealed).
   • doc/project/enforcement/check-naming.sh --base upstream/master: PASS (0 mechanical candidates in changed files).
   • doc/project/enforcement/check-architecture.sh src/inet/linklayer/ieee80211: PASS (clean architecture).
3. Direct unit tests:
   • inet_run_unit_tests -m debug -f 'Ieee80211MpduSubframeHeaderSerializer.*': PASS (Ieee80211MpduSubframeHeaderSerializer_1.test passed in 0.594s).