REV-32: GPP Section-Header parsers for MD/IN/KY/RI

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What

Adds IAB GPP US-State parsers for Maryland (24), Indiana (25), Kentucky (26), Rhode Island (27) in iabconsent. Split out of REV-28 (Minnesota + TCF 2.3, PR #4).

Design

These sections differ from the older flat US-State sections in their core field set, but use the same segment layout: section value = core segment (segment 0) + optional subsection (segment 1), split on .. The spec's "Section Header" (SectionID/Version/SubSections) is part of the GPP client-side API representation and is not serialized into the encoded consent string.

• Combined MspaMode Int(2) (0=N/A, 1=Opt-Out Option, 2=Service Provider) replaces the separate MspaOptOutOptionMode/MspaServiceProviderMode. Core ordering: MspaVersion, MspaCoveredTransaction, MspaMode first.
• MD: core omits SensitiveDataProcessing and KnownChildSensitiveDataConsents; subsection 1 is the legacy GPC subsection (carries a SubsectionType prefix).
• IN/KY/RI: core adds a single KnownChildSensitiveDataConsents Int(2); subsection 1 is Sensitive Data Consents = SensitiveDataProcessing N-Bitfield(2,8), no type prefix.
• Section IDs 24–27 added; dispatch wired through NewMspa / MapGppSectionToParser. New code isolated in mspa_us_state_v2.go; only MspaMode is added to the shared struct.

Validation

Wire format + field layout verified against the canonical IAB reference encoder/decoder and test vectors in iabgpp-es PR #106:

• MD: BQAA (default, GPC excluded), BQAA.QA (default), BVVU.YA (all fields set, GPC=true)
• IN/KY/RI: BQAA (default, sensitive excluded), BQAA.AAA (default), BVVV.kkk (all fields set, sensitive=[2,1,0,2,1,0,2,1])

Tests decode those canonical strings per state and assert the parsed values, plus an end-to-end ParseGppConsent routing test. go build + full suite green (57 checks).

Notes

• MD/IN/KY/RI specs are finalized (out of public comment Dec 2025; laws in effect). iabgpp-es PR #106 is still open (not yet merged), but it is the canonical IAB reference implementation and provides the conformance vectors used here.
• SID constants for these four use explicit values (24–27) and include UsMinnesotaSID = 23 so they don't collide with the sibling REV-28 / PR REV-28: Add Minnesota (usmn) GPP parser and IAB TCF v2.3 support #4 branch.

Tracking: REV-32.

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Superseded by #6, which consolidates this branch with its sibling into a single conflict-free, test-verified PR for REVA-165 (these two branches both declared and could not both merge). Recommend closing in favor of #6.

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Superseded by #6, which consolidates this branch with its sibling into a single conflict-free, test-verified PR for REVA-165. (These two branches both declared UsMinnesotaSID — one via iota, one explicitly as 23 — so they could not both merge to master.) Recommend closing in favor of #6. My earlier comment here was truncated by a shell-quoting bug; this is the corrected note.

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Superseded by #6, which consolidated the REV-28 + REV-32 work (MN/MD/IN/KY/RI GPP parsers + TCF v2.3) and merged to master as 997f477. Closing as superseded.