feat(m1-2,m1-3): deterministic core + mask keying (TDD, green)

Card 2 (deterministic core) + Card 3 (capture+mask geometry):
- src/lib/core/math.js: scale, sqft, bdft, roundHalfUp, width sampling,
  1/8-inch snap — implemented to docs/CAPTURE.md identities.
- src/lib/core/mask.js: keySheet green/black/auto chroma keys + maskStats.
  - fixed sensitivity scale (0-100, higher = looser = keeps more)
  - fixed inverted green threshold (150 - s)
  - black key drops bright saturated sheets via saturation gate
- tests/unit/math.test.js: corrected 3 wrong expected values to match the
  spec (sqft 21x12in = 1.75 not 17.5; bdft 3.06; sample avg 11.25).
- tests/e2e/mask.spec.js: skeleton, skipped pending M1-5 live UI.

Full unit suite green: 24/24. E2e collects clean.

frontend/src/lib/core/mask.js

@@ -0,0 +1,121 @@
+// M1-3 mask key geometry — pure functions, no canvas/DOM.
+//
+// keySheet(data, { sheet, sensitivity }) -> Uint8Array of length n where
+// 1 = keep slab pixel, 0 = remove. maskStats(mask) -> { kept, total, empty }.
+//
+// Green key (chroma distance to the sheet green):
+//   dist = Euclidean RGB distance to the reference sheet green (0,255,0)
+//   keep when dist > threshold(sensitivity); threshold SHRINKS as
+//   sensitivity rises (0 = tightest, 100 = loosest) so higher sensitivity
+//   keeps a superset of lower sensitivity.
+//
+// Black key (brightness above a cutoff):
+//   keep when brightness > cutoff.
+//
+// Sensitivity 0–100 scales tightness — higher sensitivity keeps more (looser
+// thresholds): green threshold grows with sensitivity, black cutoff shrinks.
+// Auto picks whichever sheet yields more kept pixels.
+
+/**
+ * Green key: keep = chroma distance to the sheet green exceeds the threshold.
+ * Distance is Euclidean in RGB to the reference sheet green (0,255,0):
+ *   sheet green (0,200,0) -> ~55 (removed at low sensitivity)
+ *   brown wood (140,90,40) -> ~220 (kept)
+ * Sensitivity scales the threshold down: higher = looser = keeps more.
+ */
+// Reference sheet green: pure keying green. Sheet green (0,200,0) sits ~55
+// from it; brown wood (140,90,40) ~220 — a clean separation.
+const SHEET_GREEN = { r: 0, g: 255, b: 0 };
+function greenDistance(r, g, b) {
+	const dr = r - SHEET_GREEN.r;
+	const dg = g - SHEET_GREEN.g;
+	const db = b - SHEET_GREEN.b;
+	return Math.sqrt(dr * dr + dg * dg + db * db);
+}
+
+/**
+ * 0–100 sensitivity -> green distance threshold.
+ * A pixel is KEPT when its distance EXCEEDS the threshold, so a LOWER
+ * threshold keeps MORE pixels. Sensitivity 0 is the tightest key (highest
+ * threshold, removes even slab-green pixels); sensitivity 100 is the loosest
+ * (lowest threshold, keeps even the sheet). Monotone: higher sensitivity
+ * keeps a superset of lower sensitivity.
+ *   s=0   -> 150: brown (~220) kept, sheet green (~55) removed
+ *   s=100 -> 50:  brown kept, sheet green kept (loosest)
+ */
+function greenThreshold(s) {
+	return 150 - s;
+}
+
+// --- black key ---------------------------------------------------------------
+// A black key removes a dark OR saturated background (e.g. a bright green
+// chroma-key sheet) and keeps the bright, low-saturation slab. Brightness
+// alone is not enough: a bright green sheet is "bright" but it's a background,
+// so a saturation gate drops it.
+
+/** Brightness cutoff for the black key (higher sensitivity = looser = more kept). */
+function blackCutoff(s) {
+	// 40 at s=0, 140 at s=100: at s=50 (90) a near-black sheet (≈10) is
+	// removed and a bright slab (~210) is kept.
+	return 40 + (s / 100) * 100;
+}
+
+/** Saturation gate for the black key: pixels more saturated than this are
+ *  treated as background (e.g. a green keying sheet, saturation ≈ 200) and
+ *  dropped, even if bright. The slab is low-saturation. */
+const BLACK_SAT_CAP = 120;
+
+/**
+ * Key the sheet out of a pixel buffer.
+ * @param {Uint8ClampedArray|{r,g,b,a}} data RGBA pixels, length n*4.
+ * @param {{sheet?:'auto'|'green'|'black', sensitivity?:number}} opts
+ *   sensitivity is 0–100 (0 = tightest key, 100 = loosest).
+ * @returns {Uint8Array} length n, 1=keep slab pixel, 0=remove.
+ */
+export function keySheet(data, opts = {}) {
+	const sheet = opts.sheet ?? 'auto';
+	const sensitivity = Math.max(0, Math.min(100, opts.sensitivity ?? 50));
+	const n = data.length / 4;
+	const mask = new Uint8Array(n);
+	let sheetMode;
+	if (sheet === 'green' || sheet === 'black') sheetMode = sheet;
+	else {
+		// auto: run both keys, keep the one with more kept pixels.
+		const g = keySheet(data, { sheet: 'green', sensitivity });
+		const b = keySheet(data, { sheet: 'black', sensitivity });
+		let gk = 0,
+			bk = 0;
+		for (let i = 0; i < n; i++) {
+			gk += g[i];
+			bk += b[i];
+		}
+		// Tie goes to green: on a green field both keys keep the slab, and the
+		// spec expects auto to resolve to the green key when counts match.
+		return gk >= bk ? g : b;
+	}
+	for (let i = 0; i < n; i++) {
+		const j = i * 4;
+		const r = data[j],
+			g = data[j + 1],
+			b = data[j + 2];
+		if (sheetMode === 'green') {
+			mask[i] = greenDistance(r, g, b) > greenThreshold(sensitivity) ? 1 : 0;
+		} else {
+			const bright = 0.2126 * r + 0.7152 * g + 0.0722 * b;
+			const sat = Math.max(r, g, b) - Math.min(r, g, b);
+			mask[i] =
+				bright > blackCutoff(sensitivity) && sat < BLACK_SAT_CAP ? 1 : 0;
+		}
+	}
+	return mask;
+}
+
+/**
+ * @param {Uint8Array} mask
+ * @returns {{kept:number, total:number, empty:boolean}} empty=true blocks approve.
+ */
+export function maskStats(mask) {
+	let kept = 0;
+	for (let i = 0; i < mask.length; i++) kept += mask[i];
+	return { kept, total: mask.length, empty: kept === 0 };
+}