Add Schedule 14D-9 data object for tender offer recommendations

[SurafelAnshebo]

Closes #922

Implements Schedule14D9 per the boundary agreed in #922 SC 14D-9 only, not the tender-offer side (SC TO-T/SC TO-I).

Approach

• Parser: lxml.html on the filing's primary document (SC 14D-9 has no structured XML cover page like Schedule 13D/G, so this parses rendered text directly).
• recommendation is a nullable classification ("accept" / "reject" / "neutral" / None) alongside the raw Item 4 text returns None on ambiguous/hedged board language rather than guessing, per the design note in Add data objects for tender offer filings (SC TO-T, SC TO-I, SC 14D9) #922.
• Item 4 section extraction filters out quoted cross-references before picking the real section (same failure mode as the fabricated item-anchor bug in 10-Q Part II Item 1 replaced by an MD&A cross-reference; numbered-index rows fabricate item anchors (ICE/CSCO/CTSH 10-Qs, FMCC 10-Ks) #918 a naive "first match" approach picks up a table-of-contents/cross-reference occurrence instead of the real section).
• Registered SC 14D9 / SC 14D9/A in form_map and obj() dispatch in edgar/__init__.py, plus the skill YAML (edgar/ai/skills/forms.yaml).

Ground truth

Lisata Therapeutics, Inc. (CIK 320017), SC 14D-9 filed 2026-06-10, accession 0001140361-26-024737. Hand-verified against the primary document and the board unanimously recommended shareholders accept the offer, and .recommendation == "accept" matches.

Testing

• New tests: tests/test_tender_offers.py, 11/11 passing
• Full fast suite: 2733 passed, 0 failed, 25 skipped (pre-existing) — no regressions
• ruff check / ruff format clean on new files

Marking as draft per @dgunning's note in #922 and happy to adjust the object shape/API before it grows further.

Thanks!