feat(gate-b): add product-state fixture harness for TV-3/6/7/10

Real-fixture path and harness for product-state evidence on TV-3 bdft identity,
TV-6 crop/PNG rules, TV-7 undersized warn/retake, and TV-10 rounding.

- fixtures/real/: acquisition README, placeholder metadata JSON templates
- fixtures/real-fixtures.ts: loader that decodes real PNGs, generates masks,
  runs the pipeline, fails-closed when absent or unmarked
- tv3-real.test.ts / tv6-real.test.ts / tv7-real.test.ts / tv10-real.test.ts:
  product-state tests that skip when real fixtures are absent
- Updated RESULTS.md with real-fixture harness note
- Added pngjs devDependency for PNG decoding in Node.js

Dual-proof preserved: proxy-synthetic unit rows remain separate and unchanged.
Gate B is not green; product-state rows are blocked pending Ty-supplied photos.

frontend/package-lock.json

@@ -8,6 +8,7 @@
       "name": "slabuploader-frontend",
       "version": "0.0.0",
       "devDependencies": {
+        "pngjs": "^7.0.0",
         "typescript": "^5.6.3",
         "vitest": "^2.1.4"
       }
@@ -1044,6 +1045,15 @@
       "integrity": "sha512-xceH2snhtb5M9liqDsmEw56le376mTZkEX/jEb/RxNFyegNul7eNslCXP9FDj/Lcu0X8KEyMceP2ntpaHrDEVA==",
       "dev": true
     },
+    "node_modules/pngjs": {
+      "version": "7.0.0",
+      "resolved": "https://registry.npmjs.org/pngjs/-/pngjs-7.0.0.tgz",
+      "integrity": "sha512-LKWqWJRhstyYo9pGvgor/ivk2w94eSjE3RGVuzLGlr3NmD8bf7RcYGze1mNdEHRP6TRP6rMuDHk5t44hnTRyow==",
+      "dev": true,
+      "engines": {
+        "node": ">=14.19.0"
+      }
+    },
     "node_modules/postcss": {
       "version": "8.5.28",
       "resolved": "https://registry.npmjs.org/postcss/-/postcss-8.5.28.tgz",