Add SMPTE ST 2067-201:2026 IAB support

[fschleich]

Update the IAB IMF plug-in implementation for the 2026 revision of SMPTE ST 2067-201, covering the changes observable in IMF/MXF metadata.

Parsing:

• Add IABMaxObjectCount item to IABEssenceDescriptor (Table 3).
• Add IABChannelSubDescriptor (Annex E): new class + BO, set-key and item ULs registered in StructuralMetadata, wired into HeaderPartition.

Validation (IABTrackFileConstraints, MXF path):

• Adopt MCA Content / MCA Use Class: warn when absent (should be present) and, when present, validate values and combination against SMPTE ST 377-41:2023 Tables 2, 3 and 4. Stop flagging the now-deprecated MCA Audio Content/Element Kind as missing.
• Validate IABChannelSubDescriptor required items and the IABAudioDescriptionText / IABAudioDescription MSB rule (Annex E.2.5).

Validation (IMFIABConstraintsChecker, CPL-only path):

• Apply the same MCA Content / Use Class value and combination checks so a CPL-only validation catches them when no track files are available.

Tooling:

• IMFTrackFileCPLBuilder now emits an IABSequence for IAB track files instead of failing with "only Audio/Image supported".

Misc:

• Report the constraints specification as SMPTE ST 2067-201:2026.

Tests:

• New CompleteIMP2026 IMP exercising IABMaxObjectCount and IABChannelSubDescriptor end-to-end (IAB2026CompleteIMPTest).
• CPL fixtures + IABCompositionTest cases for invalid MCA Content value, invalid MCA Use Class value, invalid combination, and a valid case.
• IMFTrackFileCPLBuilder IAB test.