Add TY2023 Form 8960 line anchors and Table 1.4 pass-through facts - #180

Implements #179: the administrative anchors for the passive pass-through NIIT base.

• Form 8960 TY2023 (SOI Pub 4801 rev 6-2026): 20 facts across 10 record sets — lines 1, 2, 3, 4a, 4b, 4c, 5d, 8, 12, 17, amounts and return counts. Headline concept for Microcosm: irs_soi.form_8960.line_4c = $109,256,984,000 (4a $1,185.6B + 4b −$1,076.4B), the net passive + rental/royalty amount in the NIIT base; 2,260,296 returns. The publisher's $1,000 line-4 rounding residual is preserved rather than smoothed — a unit test asserts 4a + 4b = 4c within it.
• Pub 1304 Table 1.4 TY2023: 160 new partnership and S-corporation net income/loss facts by AGI class (returns and amounts, cols 67–74), extending the existing Table 1.4 family to 740 facts.
• Record-set notes state explicitly that SOI's entity-side partnership tables publish no passive/nonpassive split (verified against 23pa01/04/06/10/23), making Form 8960 line 4 the sole administrative anchor for this concept.

Verification: sol-built, supervisor-verified independently — full suite 650 passed, 1 skipped, 0 failed (16m49s); governance/boundary checks, package validation, source-suite builds, and merged bundle all pass; line 4c re-derived by hand from the Pub 4801 facsimile extract.

Consumed by PolicyEngine/microcosm#722 (in flight).

Fixes #179

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