fix(scripts): pin the tsbuildinfo inside build/, assert on cache hits

Two findings from review round 1, both real.

The default incremental cache path derives from `rootDir`, so it landed at
`test-servers/tsconfig.tsbuildinfo` — beside the config, OUTSIDE `build/`.
That survives the `rm -rf test-servers/build` this project documents as the
remedy for a deleted source file, leaving tsc convinced its outputs are up
to date and emitting nothing. Pinned to `./build/tsconfig.tsbuildinfo`, and
verified that removing the output dir now forces a full re-emit.

The build is memoized per repo root; the existence assertion was memoized
with it. A later caller naming an entry the first did not would be handed a
path tsc never emitted, then fail with exactly the opaque
ERR_MODULE_NOT_FOUND the helper exists to replace. The assertion now runs on
every call.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JTHVxSu8AUgHRLvo1ntZ8H
Signed-off-by: cliffhall <cliff@futurescale.com>

test-servers/tsconfig.json

@@ -5,9 +5,14 @@
     "moduleResolution": "NodeNext",
     "outDir": "./build",
     // The script consumers rebuild this project on every run (#2111), so the
-    // up-to-date case has to be cheap. `.tsbuildinfo` lands under `build/`,
-    // which is gitignored.
+    // up-to-date case has to be cheap.
     "incremental": true,
+    // Pinned INSIDE the output dir. The default derives from `rootDir`, landing
+    // it at `test-servers/tsconfig.tsbuildinfo` — beside the config, outside
+    // `build/`. That would survive the `rm -rf test-servers/build` this project
+    // documents as the remedy for a deleted source file, leaving tsc convinced
+    // its outputs are up to date and emitting nothing.
+    "tsBuildInfoFile": "./build/tsconfig.tsbuildinfo",
     "rootDir": "./src",
     "declaration": false,
     "sourceMap": true,