[vitormattos]

Issue #95 — source-backed NT009 groups after merged #102

Continues from main at ff0a325fad59b78dc85c15154381969ab8965577; the earlier PR #102 is merged. Implements two missing Annex VI v1.04.01 groups only for the opt-in non-emitting NT009 preview.

Implemented

• IBSCBS/valores/trib/gIBSCBS/gEstornoCred (Annex VI rows 438-440): exact two-decimal vIBSEstCred/vCBSEstCred, paired validation, and mandatory caller-verified official cClassTrib.ind_gEstornoCred for inclusion. False forbids the group; true requires both values. No tax classification is inferred.
• IBSCBS/valores/trib/gIBSCBS/gPagAntecipado/refNFSe (rows 441-442): opt-in explicit list of 1-99 existing invoice references, 50 characters each. Empty list omits the group. No assumptions about taxpayer liabilities, no implicit paid invoices and no undocumented deduplication restriction.
• Exact order under gIBSCBS, updated DTO constructor with optional trailing parameters, and eleven focused PHPUnit cases for valid paths, ordering, omissions, missing official evidence, malformed amounts, 50-character keys, 99-reference bound and forbidden combinations.
• Expanded contract notes in docs/NT009-CONTRACT-IMPLEMENTATION.md.

Safety and compatibility

• Keeps DpsData, XmlBuilder::buildDps(), DpsSchemaValidator, NfseClient::emit() and Akaunting's Composer pin unchanged.
• Does not turn draft XML into fiscal emission or apply consultative Annex VIII correlations.

Verification / review

• Single Git-Signing-MCP commit, cryptographically verified with matching DCO; exact base checked.
• Source paths and cardinalities documented in checksum-pinned 450-row matrix resources/contracts/nt009-dps-v1.04.01-matrix.tsv.
• All six GitHub Actions workflows passed on commit 3932a256b9e0ae8641c0b9f6c5394023b27cde87: PHPUnit (PHP 8.2/8.3/8.4), coverage/Coveralls, Psalm, PHP CS Fixer, minimum dependencies, REUSE and commit conventions. No separate local Composer/dependency run was performed.

Remaining #95 blockers

The official portal still publishes NT009 Annex VI/VII without a confirmed deployed NT009-effective XSD for production and restricted production, or a verified activation schedule. The NT009 PDF/Annex VI gIBSCBSAjuste placement contradiction, full contract modeling, official fiscal classification dependencies and XSD/SEFIN acceptance remain open.

Do not close #95, enable issuance, or update downstream dependencies on this PR's basis.