feat(cli): atm validate-calibration — offline color classification gate

Adds `atm validate-calibration LABEL_FILE` subcommand that runs the Detector
on a set of labeled PNG frames and reports per-sample PASS/FAIL with top-3
candidate colors and RGB-distance suggestions for failures. Exits 0 on 100%
PASS, 1 on any FAIL, 2 on missing/malformed label file.

- New module src/atm/validate.py with ValidationReport + SampleRecord
  dataclasses; reuses Detector.step(frame), does not reimplement color
  classification.
- main.py: new `validate-calibration` subparser and _cmd_validate_calibration
  handler wired into the dispatch map.
- samples/calibration_labels.json seeded with 3 entries from the 2026-04-17
  incident, plus a README describing the schema.
- tests/test_validate.py covers the 3 planned cases: PASS, FAIL w/ top-3
  + suggestion, missing file (graceful error, no traceback).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

samples/calibration_labels.README.md

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+# calibration_labels.json — schema
+
+Used by `atm validate-calibration` to check that the current color calibration
+classifies known-good screenshots correctly before a live session.
+
+## Schema
+
+A JSON array of entries. Each entry:
+
+| Field      | Type    | Required | Description                                                    |
+|------------|---------|----------|----------------------------------------------------------------|
+| `path`     | string  | yes      | Path to a PNG frame (relative to CWD or absolute).             |
+| `expected` | string  | yes      | Expected color name: one of `turquoise`, `yellow`, `dark_green`, `dark_red`, `light_green`, `light_red`, `gray`. |
+| `note`     | string  | no       | Freeform annotation; shown in SUGGESTIONS output.              |
+
+## Usage
+
+```bash
+atm validate-calibration samples/calibration_labels.json
+```
+
+Exit codes:
+- `0` — every sample PASS
+- `1` — one or more FAIL
+- `2` — label file missing or malformed JSON
+
+## Adding new samples
+
+1. Find a screenshot in `logs/fires/` whose dot color you can verify by eye.
+2. Append an entry with `path`, `expected`, and an optional `note`.
+3. Re-run validation. If it FAILs, the SUGGESTIONS section will tell you the
+   RGB distance between the observed pixel and the expected color's center —
+   use that as input for `atm calibrate`.

samples/calibration_labels.json

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+[
+  {
+    "path": "logs/fires/20260417_201500_arm_sell.png",
+    "expected": "yellow",
+    "note": "first arm of SELL cycle 2026-04-17"
+  },
+  {
+    "path": "logs/fires/20260417_205302_ss.png",
+    "expected": "dark_red",
+    "note": "user confirmed via screenshot (missed live alert)"
+  },
+  {
+    "path": "logs/fires/20260417_210441_ss.png",
+    "expected": "light_red",
+    "note": "fire phase (missed live alert)"
+  }
+]