…age tensors (#1413)

Closes the `apr_diff_values_compat` invariant of `apr-cli-trace-save-tensor-v1`
at PARTIAL_ALGORITHM_LEVEL via a new `diff_05_aprt_stage.rs` include slot.

When both inputs to `apr diff --values` start with magic bytes `APRT` (the
12-byte header written by `apr trace --save-tensor`), the dispatch now
bypasses the RosettaStone whole-model walker and runs an element-wise
stage-tensor diff:
- max|diff| with index
- RMS diff
- Cosine similarity (f64-accumulated for numerical stability)
- Top-K divergences sorted by |a - b|

Both JSON and pretty text output are supported. Mismatched dim_product or
layer fields fail-fast with a diagnostic error so callers don't silently
compare incompatible stages.

## Five Whys (why now, why this scope)

1. **Why is this needed?** `apr trace --save-tensor` (PR-A #1405, PR-B #1406,
   PR-C-prep #1407) writes per-stage f32 tensors as `APRT`-prefixed files.
   Without an APRT-aware diff, layer-0 stage-by-stage element-wise
   bisection per `feedback_model_1_ships_gpu_only.md` is gated on external
   tooling — exactly the kind of muda the APR-MONO §26.8 rule forbids.
2. **Why extend `apr diff` and not write a new subcommand?** The
   `apr_diff_values_compat` invariant in `apr-cli-trace-save-tensor-v1`
   already names `apr diff --values` as the verifier. Extending the
   existing flag keeps the contract surface stable.
3. **Why an include!() file instead of inlining into diff.rs?** diff.rs
   already follows that pattern (diff_accumulator, diff_output_json_text,
   diff_04). Keeping APRT logic in `diff_05_aprt_stage.rs` lets it be
   audited / removed independently and doesn't grow the parent file.
4. **Why pin via `provenance_pin_pr_d_rev1`?** Future renames of either
   `is_aprt_stage_file` or the file path break the include!() chain;
   the pin makes that visible at test-time and forces a contract bump.
5. **Why now?** Tokenization of the 27 GB Stack v1.2 Python corpus is
   running in the background for MODEL-2 (PR #1412 merged). The SHIP-007
   PR-C-real cascade for MODEL-1 needs PR-D infrastructure ready when
   step 2 (forward_traced threading) lands. PR-D is independent and can
   merge in parallel with #1408.

## Verification

- `cargo test -p apr-cli --lib commands::diff::aprt` → 11/11 PASS
  - is_aprt_stage_file: detects/rejects/truncated/missing (4 tests)
  - compute_aprt_stage_stats: identical=zero, known max/RMS, top-K sort (3)
  - run_aprt_stage_diff: dim/layer mismatch errors, identical succeeds (3)
  - provenance_pin_pr_d_rev1 (1)
- `cargo clippy -p apr-cli --lib --no-deps -- -D warnings` clean
- `pv validate contracts/apr-cli-trace-save-tensor-v1.yaml` → 0 errors

## Contract update

`apr-cli-trace-save-tensor-v1` v1.0.0 → v1.1.0:
- New FALSIFY-APR-TRACE-SAVE-009 binding `apr_diff_values_compat` at
  PARTIAL_ALGORITHM_LEVEL with 4-line `algorithm_evidence` block citing
  this PR's unit tests.

## Ship % update

MODEL-1: ~64% → ~66% (PR-D is small but discharges 1 PARTIAL invariant
and clears infrastructure blocker for SHIP-007 step E).
MODEL-2: corpus tokenization in progress (~33h ETA).

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>