docs: add faithful RUST-SPARK-STANCE.adoc (#16)

Records the existing Idris2+Zig ABI seam (src/abi/*.idr + ffi/zig/) and
SPARK/Ada admission path honestly. Seam was present but undocumented
(silent-regress risk per estate audit). No proof escapes in the corpus;
honest gaps (Zig template placeholders, Idris2 not in CI, owed octad
invariants) recorded explicitly.

Refs hyperpolymath/standards#124

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath