feat(tests): screenshot regression + Discord review, perf-tracked

New tooling in tests/tools/screenshots/ (TypeScript via tsx):
- compare.ts: one pixelmatch engine (AA-excluded), connected-component
  "where to look" boxes, old|new+boxes|heatmap triptych, per-engine floors.
- promote.ts: churn-free updater — overwrite a baseline only when decoded
  pixels differ beyond the floor, copying CI bytes verbatim (no re-encode
  churn); pulls a CI run via `gh run download` or a local --from dir.
- post-discord.ts: always-on CI-on-main report (SHA + e2e status + the
  track-only runtime-perf table), then screenshot triptychs, batched +
  size-capped + flood-collapsed + 429-aware.
- perf-report.ts: perf table with Δ vs the previous main run (via gh).
- changelog.ts: no-build git-history baseline differ (Discord trigger B).
- noise.ts / gen-manifest.ts: calibration + manifest generation.

CI wiring:
- wasm-build.yml: post-test step runs the gate + report on the already-
  produced test-results (no extra build); report-only (continue-on-error),
  posts only on push to main, inert without DISCORD_WEBHOOK_URL.
- ci-ubicloud.yml: secrets: inherit (pass the webhook through).
- screenshot-changelog.yml: ~30s no-build changelog on baseline changes.

screenshot-manifest.json: canonical 354-name set + best-effort engine tags
(313 chromium-swiftshader / 41 firefox-llvmpipe).

Normalize scale:'device'->'css' across 18 spec files (no-op at CI DSF=1)
so committed baselines are uniformly css-scaled.

Design: CI's Linux render is the single source of truth; no pinned
container (accept rare env drift -> re-promote); dev commits via promote.
Replaces the byte-cmp compare-screenshots.sh + file-size-proxy
update-baseline-screenshots.sh (kept for now until the first re-baseline).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/package-lock.json

@@ -10,8 +10,13 @@
       "devDependencies": {
         "@playwright/test": "^1.40.0",
         "@types/node": "^24.10.1",
+        "@types/pixelmatch": "^5.2.6",
+        "@types/pngjs": "^6.0.5",
         "esbuild": "^0.28.0",
+        "pixelmatch": "^5.3.0",
+        "pngjs": "^7.0.0",
         "serve": "^14.2.0",
+        "tsx": "^4.22.4",
         "typescript": "^5.9.3",
         "yjs": "^13.6.31"
       }
@@ -484,6 +489,26 @@
         "undici-types": "~7.16.0"
       }
     },
+    "node_modules/@types/pixelmatch": {
+      "version": "5.2.6",
+      "resolved": "https://registry.npmjs.org/@types/pixelmatch/-/pixelmatch-5.2.6.tgz",
+      "integrity": "sha512-wC83uexE5KGuUODn6zkm9gMzTwdY5L0chiK+VrKcDfEjzxh1uadlWTvOmAbCpnM9zx/Ww3f8uKlYQVnO/TrqVg==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/node": "*"
+      }
+    },
+    "node_modules/@types/pngjs": {
+      "version": "6.0.5",
+      "resolved": "https://registry.npmjs.org/@types/pngjs/-/pngjs-6.0.5.tgz",
+      "integrity": "sha512-0k5eKfrA83JOZPppLtS2C7OUtyNAl2wKNxfyYl9Q5g9lPkgBl/9hNyAu6HuEH2J4XmIv2znEpkDd0SaZVxW6iQ==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/node": "*"
+      }
+    },
     "node_modules/@zeit/schemas": {
       "version": "2.36.0",
       "resolved": "https://registry.npmjs.org/@zeit/schemas/-/schemas-2.36.0.tgz",
@@ -1285,6 +1310,29 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/pixelmatch": {
+      "version": "5.3.0",
+      "resolved": "https://registry.npmjs.org/pixelmatch/-/pixelmatch-5.3.0.tgz",
+      "integrity": "sha512-o8mkY4E/+LNUf6LzX96ht6k6CEDi65k9G2rjMtBe9Oo+VPKSvl+0GKHuH/AlG+GA5LPG/i5hrekkxUc3s2HU+Q==",
+      "dev": true,
+      "license": "ISC",
+      "dependencies": {
+        "pngjs": "^6.0.0"
+      },
+      "bin": {
+        "pixelmatch": "bin/pixelmatch"
+      }
+    },
+    "node_modules/pixelmatch/node_modules/pngjs": {
+      "version": "6.0.0",
+      "resolved": "https://registry.npmjs.org/pngjs/-/pngjs-6.0.0.tgz",
+      "integrity": "sha512-TRzzuFRRmEoSW/p1KVAmiOgPco2Irlah+bGFCeNfJXxxYGwSw7YwAOAcd7X28K/m5bjBWKsC29KyoMfHbypayg==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=12.13.0"
+      }
+    },
     "node_modules/playwright": {
       "version": "1.57.0",
       "resolved": "https://registry.npmjs.org/playwright/-/playwright-1.57.0.tgz",
@@ -1317,6 +1365,16 @@
         "node": ">=18"
       }
     },
+    "node_modules/pngjs": {
+      "version": "7.0.0",
+      "resolved": "https://registry.npmjs.org/pngjs/-/pngjs-7.0.0.tgz",
+      "integrity": "sha512-LKWqWJRhstyYo9pGvgor/ivk2w94eSjE3RGVuzLGlr3NmD8bf7RcYGze1mNdEHRP6TRP6rMuDHk5t44hnTRyow==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=14.19.0"
+      }
+    },
     "node_modules/punycode": {
       "version": "2.3.1",
       "resolved": "https://registry.npmjs.org/punycode/-/punycode-2.3.1.tgz",
@@ -1557,6 +1615,40 @@
         "node": ">=8"
       }
     },
+    "node_modules/tsx": {
+      "version": "4.22.4",
+      "resolved": "https://registry.npmjs.org/tsx/-/tsx-4.22.4.tgz",
+      "integrity": "sha512-X8EX+XV4QR5xCsrgxaED954zTDfY8KqlDtskKEL0cHhyS/P8b4IFOvGDQpsC9Q1XnLq915wEfwwY/zzskCtmhg==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "esbuild": "~0.28.0"
+      },
+      "bin": {
+        "tsx": "dist/cli.mjs"
+      },
+      "engines": {
+        "node": ">=18.0.0"
+      },
+      "optionalDependencies": {
+        "fsevents": "~2.3.3"
+      }
+    },
+    "node_modules/tsx/node_modules/fsevents": {
+      "version": "2.3.3",
+      "resolved": "https://registry.npmjs.org/fsevents/-/fsevents-2.3.3.tgz",
+      "integrity": "sha512-5xoDfX+fL7faATnagmWPpbFtwh/R77WmMMqqHGS65C3vvB0YHrgF+B1YmZ3441tMj5n63k0212XNoJwzlhffQw==",
+      "dev": true,
+      "hasInstallScript": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "darwin"
+      ],
+      "engines": {
+        "node": "^8.16.0 || ^10.6.0 || >=11.0.0"
+      }
+    },
     "node_modules/type-fest": {
       "version": "2.19.0",
       "resolved": "https://registry.npmjs.org/type-fest/-/type-fest-2.19.0.tgz",