fix(producer): fail closed when no live element count is available

R4 review finding, and the comment I wrote in R3 was simply wrong: the
probe session is NOT running for every render. probeStage's needsBrowser
gate launches one only for unknown duration, unresolved compositions, or
specific media cases — and hasRuntimeInsertedMedia matches only
createElement("video"|"audio"), never createElement("span"). So the exact
shape that motivated the live-DOM fix (a known-duration, media-free
caption comp building thousands of nodes in script) gets NO probe, falls
back to the static source scan, reads as ~2 elements, and could enter the
applied cohort at 40k live nodes. The R3 fix measured the right thing but
only for the population that already had a probe.

Now the count carries provenance and the band fails closed:

- resolveCompositionElementCount returns { count, source: "live" |
  "static" }. Only "live" — an actual DOM measurement — may open the band.
- resolveDeShortBand gains a third decisive outcome, "unmeasured", for
  the static case. It deliberately does NOT report skipped_elements: a
  static undercount is not a real oversize observation, and putting it in
  the control arm would contaminate the DiD just as putting it in the
  treatment arm would. Neither cohort; never routes.
- composition_element_count_source ships alongside the count, so the
  fleet rate of "static" sizes the population a future
  conditional-probe-launch would unlock — which is the data PR B needs to
  decide whether that launch cost is worth paying.

Regression coverage walks the real chain rather than a full render, using
the production functions in pipeline order: probeRequiresBrowser (newly
extracted from the inline needsBrowser expression, so the gate is
testable at all) returns false for the caption-comp shape → the resolver
reports static and a count under the ceiling → the band reports
unmeasured, not applied. Fault injection confirms it bites: removing the
one guard line fails exactly these three tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packages/cli/src/commands/render.ts

@@ -1433,6 +1433,7 @@ function trackRenderMetrics(
     deWorkerInversion: perf?.drawElement?.workerInversion,
     dePreInversionWorkers: perf?.drawElement?.preInversionWorkers,
     compositionElementCount: perf?.drawElement?.compositionElementCount,
+    compositionElementCountSource: perf?.drawElement?.compositionElementCountSource,
     deShortBand: perf?.drawElement?.shortBand,
     deParallelRouter: perf?.drawElement?.parallelRouter,
     dePreRouterWorkers: perf?.drawElement?.preRouterWorkers,