engine: vdf find_slot_table under-counts slot entries on risk2.vdf and SCEN01.VDF

[bpowers]

Problem

src/simlin-engine/src/vdf.rs::find_slot_table uses a min_stride >= 4 rule when scoring candidate slot tables. This rule rejects valid leading-extra-slot layouts that appear in certain edited or large VDF fixtures, causing the Rust parser to under-count slot entries relative to the reference implementation (tools/vdf_xray.py).

Concrete cases:

• test/bobby/vdf/econ/risk2.vdf -- Rust returns 46 entries; Python returns 106 entries.
• test/metasd/WRLD3-03/SCEN01.VDF -- Rust returns 404 entries; Python returns 420 entries.

The block1[7] header word in both files matches Python's slot count exactly, which is an independent cross-check that Python's count is correct and Rust is wrong.

Why it matters

Correctness of the VDF parser downstream. Under-counting slot entries loses variable data and blocks further name-to-OT mapping work -- notably, closing this gap should unblock downstream mapping improvements on SCEN01.VDF (WRLD3-03).

Component

src/simlin-engine/src/vdf.rs (find_slot_table)

Current workaround

src/simlin-engine/tests/vdf_structural_invariants.rs contains a new test test_block1_word7_matches_slot_count_within_small_delta that asserts block1[7] ~= slot_count across VDF fixtures. The two fixtures above are exempted via a rust_slot_table_undercount_known() helper. Closing the parser gap lets that exemption be removed.

Possible approaches

• Investigate the min_stride >= 4 heuristic: understand which legitimate layouts it rejects and whether a weaker stride check (e.g., allowing a leading extra slot that doesn't match the rest of the stride) preserves the signal that motivated the rule in the first place.
• Compare the candidate scoring against tools/vdf_xray.py's slot-table detection and port the Python heuristic where it diverges.
• Relates to (but is distinct from) tech-debt entry 19 "Rust VDF Parser Boundary Parity" in docs/tech-debt.md, which covers slot-table offset disambiguation in SCEN01.VDF rather than entry-count scoring.

Context

Identified while adding VDF format structural invariant tests. The test exemption is in place so we can land the invariant test without blocking on this fix, but the exemption should go away once the parser matches Python.