feat(#25): Gate B pricing and thickness band (TV-8, TV-9)

TV-8 pricing:
- computeRecommendedPrice(bdft, pricePerBdft) returns bdft × price/bdft rounded half-up to 2 decimals
  0.96 bdft × 12.50/bdft → 12.00
- computeSlabPrice handles three paths:
  1. Has rule, no override → recommended price, source 'recommendation'
  2. Has rule, override → override stored (150.00), source 'override'
  3. No rule, manual → manual stored (99.99), recommendedPrice null

TV-9 thickness band:
- findThicknessBand(thicknessIn, ranges) rounds to 1/8" then matches [min, max) half-open ranges
- 1" → 1"–1 1/2" (rounds up, label from synced Woo attribute, not hardcoded)

Test output (all 29 tests green, 6 new):
- tv8-pricing.test.ts: 6 passed (recommended, override, no-rule, rounding)
- tv9-thickness-band.test.ts: 6 passed (1"→1"–1 1/2", 0.75"→3/4"–1", 0.875"→3/4"–1", 1.5"→1 1/2"–2", null for out-of-range, rounding to 1/8")

frontend/src/__tests__/fixtures/pricing.ts

@@ -0,0 +1,20 @@
+/** TECH-SPEC §8 TV-8 pricing fixture. */
+export function tv8Fixture(): {
+  bdft: number;
+  pricePerBdft: number;
+  expectedRecommended: number;
+  overridePrice: number;
+  expectedOverride: number;
+  manualPriceNoRule: number;
+  expectedManual: number;
+} {
+  return {
+    bdft: 0.96,
+    pricePerBdft: 12.50,
+    expectedRecommended: 12.00,
+    overridePrice: 150.00,
+    expectedOverride: 150.00,
+    manualPriceNoRule: 99.99,
+    expectedManual: 99.99,
+  };
+}