test: testing overhaul -- CI e2e gates, parallel suites, generated matrices, mutation testing (#215)

Closes the "e2e never runs in CI" hole. Adds per-PR e2e smoke gate,
nightly full-suite workflows, parallel vitest forks (per-fork DBs),
Playwright parallel/serial/visual projects against production builds,
metadata-generated test suites (drift guards, hostile inputs, format
matrix, pairwise settings, property-based fuzz), Stryker mutation
testing, Schemathesis API fuzz, coverage ratchet, and fixes for three
session-poisoning bugs that caused 200+ serial-bucket failures.

Bug fix included: favicon/split/bulk-rename could hang clients forever
when ZIP streaming failed after reply.hijack().

packages/image-engine/package.json

@@ -9,7 +9,8 @@
     "lint": "biome check src/",
     "typecheck": "tsc --noEmit",
     "test": "vitest run",
-    "clean": "rm -rf dist"
+    "clean": "rm -rf dist",
+    "test:mutation": "stryker run"
   },
   "dependencies": {
     "@snapotter/shared": "workspace:*",
@@ -17,6 +18,8 @@
     "sharp": "^0.34.5"
   },
   "devDependencies": {
+    "@stryker-mutator/core": "^9.6.1",
+    "@stryker-mutator/vitest-runner": "^9.6.1",
     "typescript": "^5.7.0",
     "vitest": "^3.2.6"
   },

packages/image-engine/stryker.config.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "https://raw.githubusercontent.com/stryker-mutator/stryker-js/master/packages/api/schema/stryker-core.json",
+  "testRunner": "vitest",
+  "plugins": ["@stryker-mutator/vitest-runner"],
+  "mutate": ["src/**/*.ts", "!src/types.ts", "!src/index.ts"],
+  "incremental": true,
+  "incrementalFile": "reports/stryker-incremental.json",
+  "reporters": ["html", "clear-text", "progress"],
+  "htmlReporter": { "fileName": "reports/mutation/mutation.html" },
+  "thresholds": { "high": 80, "low": 60, "break": null },
+  "tempDirName": ".stryker-tmp",
+  "ignoreStatic": true
+}