fix(test-runner): cancellation terminates the run; win32 kills the whole tree

Installing SIGINT/SIGTERM forwarders suppresses Node's default
terminate-on-signal, so a cancelled run killed the current child and
kept LAUNCHING shards — observed as paid runs continuing to burn API
spend after Ctrl-C (codex adversarial, repro'd ALIVE_AFTER_SIGTERM).
The first signal now also schedules the parent's own exit after the
children's SIGKILL grace, and both shard pools consult
isTerminationRequested() before taking new work. On win32,
killProcessGroup uses taskkill /T /F — detached:true creates no
killable group there, and a bare child.kill orphaned every grandchild
(ports, locks, and the inherited pipes that kept close from firing).
Also: the tree-mutating serial shard prints dirty generated artifacts
when it dies mid-regeneration, and --shard CI-matrix mode gets the
same size-scaled wall deadline as full-suite mode.

scripts/test-paid-shards.ts

@@ -59,6 +59,7 @@ import {
   exactTestFileSelectors,
   forwardAndClassify,
   installChildSignalForwarding,
+  isTerminationRequested,
   killProcessGroup,
   strictTestExitCode,
 } from './test-strict-output';
@@ -517,6 +518,10 @@ export async function runPaidShards(
   let next = 0;
   const worker = async (): Promise<void> => {
     while (true) {
+      // Cancellation (SIGINT/SIGTERM) must stop the RUN: the signal
+      // forwarders kill in-flight children, and this guard stops the pool
+      // from launching replacement shards that would keep burning API spend.
+      if (isTerminationRequested()) return;
       const index = next;
       next += 1;
       if (index >= shards.length) return;