Add Lower Saxony NEFG acceptance case and two parser robustness fixes

Second §-structured Landesrecht case for wave six: the Lower Saxony ELER Support
Act (NEFG, unchanged since 2022, derived from the two-column Nds. GVBl 2022 No. 33)
plus the amending act of 28 Jan 2026 (Nds. GVBl 2026 No. 10). Five of seven
commands apply — both "erhält folgende Fassung" re-enactments (§ 1(1), § 2), the
citation replacement in § 1(5), the words insertion in § 6(1), and the § 13 → § 14
renumbering.

- LandesRechtTextParser: a norm head no longer accepts a title ending in a full
  stop, so a cross-reference sentence at line start ("§ 7 GAPInVeKoSG findet
  entsprechend Anwendung.") is not misread as a norm head "§ 7" (which had made the
  monotonicity check swallow the norms in between).
- TextBereiniger: strip the Lower Saxony GVBl running footer and publisher line.
  The footer sat between two enumerated items and otherwise attached to the
  preceding re-enactment quote, defeating its end-of-sentence anchor.
- Add sampledata/Niedersachsen/NEFG-alt.txt and EndToEndTest.nefgAcceptance.

Two insertion forms remain documented residues (manuell prüfen): inserting a
§-block with a space-separated sub-number ("§ 2 a") and a chapter-scoped §-block
insertion ("In Kapitel 4 wird nach § 12 … angefügt").

221 tests green; federal, Bavarian and Saxon pinned numbers unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Change-Id: Ie23eee1f596ec8eecc6a73f6ce585a22e73ec6c9

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