ABI Layer 4: prove the FFI result-code seam is injective + faithful (#177)

## Summary

Layer 4 (seal the ABI↔FFI seam): proves the FFI result-code encoding
(`resultToInt`) is a **sound, lossless wire encoding** — decoder
`intToResult` with `resultRoundTrip`, from which `resultToIntInjective`
is derived. Distinct ABI outcomes never collide on the C wire.
Complements the structural `abi-ffi-gate.py`.

New module `*.ABI.FfiSeam` (imports `Types`). Positive + non-vacuity
controls.

## Testing
Idris2 0.7.0 `--build` → exit 0, zero warnings. Adversarial rejection
confirmed. `build/` removed. No `believe_me`/`postulate`/`sorry`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01A6PSzJWpRxtzGDjUCEh7Mx

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_Generated by [Claude
Code](https://claude.ai/code/session_01A6PSzJWpRxtzGDjUCEh7Mx)_

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Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath