[P2] ephapax: 11 partial + add ABI seam (Rust/SPARK NON-COMPLIANT)

[hyperpolymath]

35k-LOC compiler: 11 partial in Idris2; bare Rust, no seam, 49 unsafe. Tighten totality + add ABI seam for type-checker invariants + stance doc.

Sub-issue of #124. PRs Refs hyperpolymath/standards#124. Joint-close only on explicit agreement.

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