feat(apr-cli): CRUX-B-20 apr diff --quant-roundtrip per-tensor error report

[noahgift]

Summary

Closes the per-tensor quantization-error parity gap with llama.cpp's
`llama-quantize-stats -v`:

```
apr diff REF.safetensors QUANT.safetensors --quant-roundtrip [--json]
[--threshold 0.95] [--no-threshold]
```

For every tensor present in both files, computes RMSE / cosine / max_abs
error pairwise, sorts by RMSE descending, and emits a TSV (default) or
JSON (`--json`) report. Each row carries a green/yellow/red verdict per
the contract bucketing (cosine ≥ 0.999 / ≥ 0.99 / else).

Threshold gate (contract invariant): when ANY tensor's cosine falls
below `--threshold` (default 0.95), the command exits non-zero unless
`--no-threshold` is set. Useful as a CI gate against quant regressions.

Implementation

• New module `crates/apr-cli/src/commands/diff_quant_roundtrip.rs`:
  • `error_metrics(reference, quantized) → (rmse, cosine, max_abs)`
    pure f64-accumulated math, deterministic
  • `build_report(ref_path, quant_path, threshold) → Report` safetensors
    loader supporting F32 / F16 / BF16
  • `render_tsv` / serde JSON output paths
• Existing `apr diff` clap variant extended with three flags
  (`--quant-roundtrip`, `--threshold`, `--no-threshold`)
• Dispatch in `dispatch.rs::dispatch_quant_roundtrip` returns
  `CliError::ValidationFailed` on threshold breach → exit code 5

Tests

8 in-tree tests under `cargo test -p apr-cli --lib commands::diff_quant_roundtrip`:

• Metrics: identity, anti-parallel, orthogonal, empty, small-error
• Verdict bucket boundaries (green/yellow/red at 0.999/0.99)
• FALSIFY-CRUX-B-20-001: rows sorted rmse DESC; every row carries
  rmse/cosine/qtype/numel/max_abs/verdict
• FALSIFY-CRUX-B-20-002: threshold gate flips on cosine < threshold

Live e2e verified on built binary:

```
$ apr diff ref.safetensors quant.safetensors --quant-roundtrip --json

exit 0 when all tensors green; exit 5 (ValidationFailed) when any red

```

Contract promotion

`contracts/crux-B-20-v1.yaml`:
v1.1.0, status: draft, intake_status: partial
→ v1.2.0, status: partial_algorithm_level, intake_status: supported

FALSIFY-CRUX-B-20-001/002 ship as in-tree.
FALSIFY-CRUX-B-20-003 (parity with `llama-quantize-stats` on a golden
GGUF model) remains pending — separate ticket; needs routing through
`format::gguf::dequant.rs`.

3-surface drift compliance

• `commands_enum.rs` — 3 new fields on existing Diff variant
• `dispatch.rs` — new `dispatch_quant_roundtrip` helper + Diff pattern updated
• `contracts/apr-cli-commands-v1.yaml` — `diff` description mentions B-20
• 3 test sites updated to construct new clap shape
• `cli_commands.rs` 8/8 still green

Out of scope

• GGUF Q4_K / Q6_K reference path (separate ticket)
• Parity sha256 against `llama-quantize-stats` golden tensors

🤖 Generated with Claude Code