[P0] ambientops: add RUST-SPARK-STANCE + kill SPARK theatre

[hyperpolymath]

19 SPARK_Mode files, only ~8 carry Pre/Post/Contract_Cases → ~11 contract-less (false confidence on 'reversibility guarantees'). Add stance doc; add contracts to the 11 or demote SPARK_Mode + move logic to Idris2 seam.

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

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[hyperpolymath]

PR hyperpolymath/ambientops#48 opened (Refs, not Closes) — adds RUST-SPARK-STANCE.adoc.

Audit reconciliation: 'SPARK_Mode On, zero contracts' was a body-vs-spec miscount. Safety-critical specs are heavily contracted (snapshot_manager.ads 25, safety_boundary.ads 15, safety_invariant.ads 14; 8/10 specs contracted). ambientops already had Idris2 ABI seam + Zig FFI + spark build mode — structurally compliant; only the stance doc was missing (now added).

Remaining (keeps #127 open): genuine engineering, not doc — backend_interface.ads abstract interface needs Pre'Class/Post'Class for gnatprove to propagate obligations to backend_guix/backend_nix; plugin_registry.ads weak (1 contract). reversibility_types.ads deliberate SPARK_Mode(Off) is honest/documented — leave. Stance-doc arm done; contract-discharge arm tracked here.