fix(#4012): make the artifact say whether the reporter ever loaded

Down to 3 remote failures, all one chain. The explicit no-events reporting is
working — the diagnostic now states the events file does not exist, instead of
silently printing the generic message. But it then ASSERTED a cause: "the child
was killed before the reporter wrote even one event (process/spawn startup
stall, not a test hang)". That was a guess dressed as a finding, and the fixture
contradicts it: it starts a real test, so test:start should fire in milliseconds
against a 2000ms budget.

Two hypotheses remained and I could not separate them locally, because the local
test runner is hook-blocked here: either the custom reporter never LOADS in the
child, or it loads and no event reaches it before the SIGKILL.

Rather than guess a third time, the artifact now answers it. The reporter
appends a reporter:init line as its first action, before consuming anything, so
the file's contents discriminate: absent means the reporter never loaded;
init-only means it loaded and saw no test events; init plus events means it
works. The diagnostic has a branch for each and, where the cause is genuinely
unresolved, names both possibilities instead of picking one.

Also passes the reporter as a file:// URL via pathToFileURL. Node documents the
--test-reporter value as an import()-style specifier, and a bare absolute path
is not a portable one — notably on Windows. That is a correctness fix whichever
hypothesis holds, and it is a live candidate for the first. FIXED_OVERHEAD is
derived by reducing over the actual argv strings, so the longer URL is accounted
automatically.

Verified by execution, not assumption: composing the reporter against an EMPTY
event stream writes exactly one line, the init marker. That is the whole point
of the marker, so it is pinned by a test rather than left to inspection.

T1 stays red and untouched.

Verification runs on the remote runner.

Refs #4012

scripts/lib/ndjson-reporter.cjs

@@ -57,6 +57,23 @@
 // never leaves more than one dangling unparsable trailing line.
 module.exports = async function ndjsonEventReporter(source) {
   const eventsPath = process.env.GSD_RUN_TESTS_EVENTS_FILE;
+  // #3889: an init marker, written as this reporter's FIRST action — before
+  // the `for await` loop even begins consuming the event stream — so the
+  // events file's mere existence (and its exact contents) can distinguish
+  // "the reporter module never loaded in the child at all" (file absent)
+  // from "it loaded fine but no test:start reached it before the kill"
+  // (file contains only this one line) from "it's working" (file contains
+  // more than this line). Same appendFileSync durability rationale as every
+  // other write in this file: synchronous and unbuffered, so it survives an
+  // uncatchable SIGKILL landing a moment later.
+  if (eventsPath) {
+    try {
+      require('fs').appendFileSync(eventsPath, `${JSON.stringify({ type: 'reporter:init', ts: Date.now() })}\n`);
+    } catch {
+      // Best-effort, same as every other write below — must never crash the
+      // test run this reporter is only observing.
+    }
+  }
   for await (const event of source) {
     if (!eventsPath) continue; // no destination configured — nothing to record
     if (