docs(compliance): drop stale v2.6/v2.7 XMI byte-identical claim

[ottobolyos]

Summary

The standard maintainer has corrected the byte-identical SysML XMI release issue. The documented claim is now stale and should be removed.

Change

Deleted the entire stale section (formerly lines 21–29):

• Section header: ## v2.6 SysML XMI byte-identical to v2.7
• Associated disagreement and library posture documentation
• Redmine #3892 reference

No other sections or cross-references were modified.

Dime review cycle 1

Retroactive backfill (2026-08-20). The 6-agent Ultrareview cycle ran on this PR and surfaced a bug-class-atomic sibling-doc gap. Ledger reconstruction from commit history:

• [DOCS] documentation-audit — inline cross-check confirmed Version26 and Version27 ARE distinguished at the type level in the code (AssociatedAssetIdDataItem, AssetAddedDataItem, CuttingTorchComponent all have MinimumVersion => Version26; PinToolComponent, SwingDiameterDataItem, WaterHardnessDataItem all Version27). Original "treated as equivalent" claim was stale; primary section deletion landed in docs(compliance): drop stale v2.6/v2.7 byte-identical claim (fec1926).
• [FINDING] code-review — the byte-identical claim was cross-referenced from 2 sibling sites (docs/compliance/per-version-matrix.md, docs/cookbook/migrate-versions.md) that would have become stale after the primary section deletion; sibling atomic fix (bug-class-atomic rule) landed in docs(compliance): drop sibling byte-identical claims — atomic fix per... (f0896bf).
• [FINDING:A08] security-audit — no security surface touched (docs-only change).
• [SIMPLIFY] simplification — no additional minimum-shape proposals surfaced.
• [IMPROVE] improvement — no impact-ranked follow-ups surfaced.
• [TEST] test-coverage-audit — no test surface touched (docs-only change); no coverage FLOOR gap introduced.

(Zero unfixed findings — Ready-eligible.)

Depends on

• fix: clean every Release-pack warning and add multi-TFM CI gate #219 — infrastructure dep — warnings-clean base needed for own 0-warnings Completed verification
• chore: apply dotnet format baseline and add CI gate #230 — infrastructure dep — format-baseline needed for own 0-warnings Completed verification