…9200)

Reduced to the one fix of the original three that is still needed. The other two
landed by other routes while this sat: the orphan-scan flake was fixed by #9280,
and the extensionless `./mmproj-fallback` import is already `.ts` on main. Both
are verified identical to main here rather than assumed.

On #9280 taking a different approach: it pins `_pid_parent_is_alive` to a
constant for the whole test, where this PR stubbed it only for the invented PID
and delegated every other PID to the real implementation. That is a real
difference in coverage, so I checked whether main lost anything before dropping
mine. It did not: `test_reap_recorded_pid_spares_live_server` in
test_llama_cpp_wait_for_vram_settle.py exercises the ownership gate for real,
against a process the test genuinely owns. So the gate is still covered and
#9280's version stands.

What is left, and why it is still needed
------------------------------------------------------------------------
`saveMarkdownAsProjectSource` fires `watchIngestion` without awaiting it, and
that watcher polls for up to 300 seconds. Watchers from earlier tests therefore
outlive them and keep calling the shared fetch stub, which by then belongs to
whichever test is running now. The stub answered any `/jobs/` URL and the uploads
shared a filename, so a stale watcher emitted a toast byte-identical to the one
the running test was waiting on, under a different project id.

The test then waited on the toast and read the announce count without polling.
The watcher toasts and announces with nothing awaited between them
(save-markdown-source.ts, `toast.error(...)` then
`announceProjectSourcesUpdated(...)`), so a tick landing in that window saw 1
instead of 2.

Each job handler now answers only its own job id and 404s anything else, so a
stale watcher dies quietly instead of borrowing the running test's answers. The
save under test gets its own filename, and the test polls on the announce, which
the watcher does last, then asserts the toast synchronously.

Verified
------------------------------------------------------------------------
Gap sweep, inserting a 150 ms delay into the source between the toast and the
announce to widen the window the race needs: main's version of the test fails 6
of 6 runs, this version passes 6 of 6. Both pass 6 of 6 on an idle box with no
delay, which is why this reads as an intermittent CI failure rather than a
reproducible one.

Mutation-tested: deleting `announceProjectSourcesUpdated` from the watcher fails
this test (9 passed, 1 failed), so it still proves what it did before rather than
having been loosened into passing.

Full frontend suite: 4087 passed, 0 failed.