twfe: clear stale TWFE-rejection claims in CHANGELOG release notes

CI review (R4) flagged that the new top entry says TWFE HC2/HC2-BM is
supported, but two older bullets in the same [Unreleased] block (DiD-
absorb and MPD-absorb entries) still claim "TwoWayFixedEffects
rejection remains as a follow-up" / "remain as follow-ups". Those
sentences were factually correct when those PRs landed but become
contradictory once Gate 1 ships in the same release.

Replace both with cross-references to the Gate 1 top entry, noting that
TWFE uses a separate full-dummy branch (no fixed_effects= equivalent
inside TWFE) rather than the absorb→fixed_effects parameter swap used
by DiD/MPD.

Per feedback_changelog_accuracy_fixes.md: scanned all CHANGELOG bullets
for similar stale claims; the remaining matches are unrelated entries
(Spillover, BaconDecomposition, Conley waves) that don't reference TWFE
rejection.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber