Settle the render window on the time selection and delete the experiment that proved it

The millisecond floor lives in the custom-bounds field, not the engine, so
RENDER_BOUNDSFLAG=2 is now the only bounds mode: the two-position type, the
console verdict and the STARTPOS/ENDPOS drift probe all go. capture.cpp 697 -> 622.

docs/TODO.md

@@ -765,20 +765,26 @@ these refusals — but it means a future one- or two-frame refusal may be ours,
 the tolerance was not widened on speculation. Widening it is a precision-invariant
 decision, not a bug fix.
 
-## `capture.cpp` is over the ~600-line ceiling — documented, not split mid-experiment
+## `capture.cpp` is over the ~600-line ceiling — the seam is identified, taking it is blocked
 
-**Context.** The bounds-channel live experiment (`RenderBoundsChannel`, this same
-section above) added the time-selection guard/read-back plumbing and the always-on
-verdict print to `OfflineRenderBackend::capture`, landing the file at 697 lines against
-root `CLAUDE.md`'s ~600-line ceiling. The named seam: the drift/verdict instrumentation
-block (`ScopedTimeSelection`/read-back/drift-report/verdict-print, roughly
-`capture.cpp:449-655`).
+**Context.** Removing the settled bounds experiment's instrumentation (the console
+verdict and the three-checkpoint `RENDER_STARTPOS`/`ENDPOS` read-back) brought the file
+from 697 to **622 measured lines**, against root `CLAUDE.md`'s ~600-line ceiling. The
+seam that entry originally named is gone with the instrumentation; nothing left in the
+file is bisectable without cutting load-bearing why.
 
-**Deferred, not silent.** ≈60 of the added lines are temporary probe instrumentation
-with a known removal date (the experiment closes when `docs/VERIFICATION.md` §Capture
-range and bounds comes back), and splitting the file mid-experiment risks moving the
-exact code the smoke run is measuring. Split after the experiment closes, onto the seam
-named above.
+**The remaining seam is a real responsibility boundary**, and the file header already
+names it as two things: `OfflineRenderBackend::capture` (the offline render driver)
+versus the four helpers BOTH backends share — `makeUniqueTag`, `captureNameFor`,
+`collapseCapturedFileToMono`, `stampCaptureSample` — consumed by `capture_batch`,
+`capture_orchestrator`, `capture_realtime_shell`, `capture_realtime_finalize` and
+`render_in_place`. Lifting those four into their own TU takes the driver under the
+ceiling and gives the cross-backend steps their own home.
+
+**Why not taken.** `src/shell/capture/` has no `CMakeLists.txt` of its own — its sources
+are listed in `src/app/CMakeLists.txt`, so a new TU needs an edit there. Forcing the
+four helpers into an existing TU instead (orchestrator, realtime finalize) would put
+them in a wrong home to dodge one build-file line, which is worse than the overshoot.
 
 ## bext TimeReference read-back is not a floor detector (dead end, recorded so it is not re-litigated)