chore(scripts): build test-servers unconditionally from one shared helper

Five `scripts/*.mjs` consumers each carried their own `ensureTestServer()`
that returned early when `test-servers/build` was already on disk. Presence
is not freshness: once that directory exists — which it does on any machine
that has run the smokes once — an edit to `test-servers/src` was never
picked up again, and the smoke went on driving the previously-built fixture.

The failure is silent and misattributed. It does not read as "your fixture
is stale"; it reads as a product bug in whatever the smoke is checking.

`scripts/lib/ensure-test-servers.mjs` is now the one place that build is
produced for a script consumer, and it builds unconditionally — memoized
once per process per repo root, so pack-and-verify's two asks stay one tsc
pass while a fresh run always rebuilds. Callers name the entries they
depend on and get one actionable failure message, replacing five that had
drifted. pack-and-verify also stops shelling out to `npx tsc`, which is
what made it the one script unrunnable on Windows (#1939).

`test-servers/tsconfig.json` gains `incremental` so the up-to-date case is
cheap: 3.0s cold, 1.6s incremental here.

Unconditional emit is still not a clean — `--noCheck` leaves the stale .js
of a deleted src file behind — which is unchanged from before and recorded
in the helper's header and AGENTS.md.

Closes #2111

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

@@ -4,6 +4,10 @@
     "module": "NodeNext",
     "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.
+    "incremental": true,
     "rootDir": "./src",
     "declaration": false,
     "sourceMap": true,