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.

src/core/capture/CLAUDE.md

@@ -51,8 +51,8 @@ Detail specific to these pure modules:
 - `capture_paths` — the REAPER-free path arithmetic behind offline capture: bank-subfolder + unique-filename derivation (`deriveBankPaths`, forward-slash form, no filesystem touch), the absolute-render-dir vs. project-relative-index-path split (`BankPaths`), the persist-side inverse (`resolveBankFile`, `projectDirOfRpp`), the Save-As bank-relocation plan (`deriveRelocationPlan`), and the GUID-primary project-identity classifier (`classifyProjectTransition` → `NoOp`/`Load`/`SaveAsRelocate`) the persist-poll timer drives.
 - `capture_name` — the REAPER-free composition of one capture's label + file-stem base from its source-track name(s), a local-calendar discriminator (`MM-DD HHMM`, from the shell's clock read), and an optional batch ordinal. The label and the stem deliberately diverge: the stem still passes through `capture_paths::sanitizeStem` (so a name that sanitizes to nothing files as `capture`), while the label keeps the source name verbatim. Stem uniqueness stays entirely `makeUniqueTag`'s — this module never disambiguates.
 - `insert_plan` — the REAPER-free logic behind the `insert` shell (M6): computes the `InsertMedia` `mode` bitmask from an `InsertOptions` struct (placement target, tempo-conform ratio, preserve-pitch flag), guaranteeing the &4 stretch-to-time-selection bit is never set and that no tempo bits are set when `conform == None`.
-- `render_settings` — the REAPER-free logic behind the capture action family: `SourceMode` → `RENDER_SETTINGS` bit mapping, `P_RAZOREDITS` string parsing + range-union bounds, razor-else-time range inference, the FX-scope bypass plan (`fxBypassPlanFor`), the bounds channel a capture hands its window over on (`RenderBoundsChannel`/`renderBoundsFlagFor`/`renderBoundsChannelLabel`), the tail-mode → `RENDER_TAILFLAG`/`RENDER_NORMALIZE`/`RENDER_TRIMEND` mapping (`tailRenderSettingsFor`) and its realtime-window analog (`realtimeRecordWindowEnd`), the capture-action taxonomy table (`captureActionTable`) `main.cpp` iterates to register the CAPTURE_ITEM/CAPTURE_TRACK family, and `renderSourceLabel` (the source named in the offline backend's bounds refusal).
-- `render_window` — the REAPER-free frame arithmetic behind exact capture bounds: `frameCountFor` (the frame count a project-time window occupies at the project rate — the number the offline backend checks the rendered file against before landing it, so a render that printed something other than the window is refused rather than banked), `renderHonoredBounds` (the gate's verdict and the sole home of its one-frame tolerance, which is empirical rather than proven — the header states which renderer models it covers and which it does not), and `itemExtentPrintsWindow`, the predicate `render_settings::sourceModeForScope` consults to decide whether REAPER's selected-items render source can express a requested window at all. It also owns the short-render diagnostics: `msFlooredEndFrameCount` (the frames a window holds with its end floored to the millisecond — the shape two live short renders matched, quoted by the refusal as a count coincidence and nothing more), `describeBoundsDrift` (the sentence the offline backend prints when a channel's stored bounds do not read back as they were written), `isOnMillisecondGrid` (whether an observed edge can speak to a rounding question at all — an on-grid edge cannot), and `describeBoundsExperiment` (the always-printed verdict naming which bounds channel carried a capture's window and what the landed file measured).
+- `render_settings` — the REAPER-free logic behind the capture action family: `SourceMode` → `RENDER_SETTINGS` bit mapping, `P_RAZOREDITS` string parsing + range-union bounds, razor-else-time range inference, the FX-scope bypass plan (`fxBypassPlanFor`), the one bounds mode a capture hands its window over on (`kRenderBoundsTimeSelection`) and the tail bit paired with it (`kTailFlagTimeSelection`), the tail-mode → `RENDER_TAILFLAG`/`RENDER_NORMALIZE`/`RENDER_TRIMEND` mapping (`tailRenderSettingsFor`) and its realtime-window analog (`realtimeRecordWindowEnd`), the capture-action taxonomy table (`captureActionTable`) `main.cpp` iterates to register the CAPTURE_ITEM/CAPTURE_TRACK family, and `renderSourceLabel` (the source named in the offline backend's bounds refusal).
+- `render_window` — the REAPER-free frame arithmetic behind exact capture bounds: `frameCountFor` (the frame count a project-time window occupies at the project rate — the number the offline backend checks the rendered file against before landing it, so a render that printed something other than the window is refused rather than banked), `renderHonoredBounds` (the gate's verdict and the sole home of its one-frame tolerance, which is empirical rather than proven — the header states which renderer models it covers and which it does not), and `itemExtentPrintsWindow`, the predicate `render_settings::sourceModeForScope` consults to decide whether REAPER's selected-items render source can express a requested window at all. It also owns the one short-render diagnostic: `msFlooredEndFrameCount` (the frames a window holds with its end floored to the millisecond — the shape two live short renders matched on the retired custom-bounds mode, quoted by a refusal as a count coincidence and nothing more) and `isOnMillisecondGrid`, the whole-millisecond tolerance that count depends on.
 - `track_topology` — the REAPER-free folder arithmetic over a project's flat `I_FOLDERDEPTH` delta list: `directChildIndices` names a folder parent's DIRECT children, the set `shell/capture/render_isolation` silences so a ranged item capture does not print its track's children. Grandchildren are excluded by construction — they reach the parent only through the child that owns them.
 - `tail_control` — the REAPER-free logic behind the docked `bank_panel`'s tail-mode toggle: the cycle order (None → Auto → Manual → None), the Manual-length clamp/scroll-wheel fine-adjust (`clampManualMs`/`adjustManualMs`, 250 ms/notch, 2000 ms default), the toggle's label text (e.g. "Tail: Manual 2.0s"), and the `TailSetting` JSON round-trip persist stores per-project.
 
@@ -79,12 +79,11 @@ Detail specific to these pure modules:
   that with a transient silencing (`shell/capture/render_isolation`) whose child-set
   walk lives here in `track_topology`; the item-vs-track asymmetry behind it is in
   `src/shell/capture/CLAUDE.md`.
-- **The render window floors to the millisecond at render time.** Measured cause,
-  why two bounds channels exist, and the live experiment: `render_settings.h`'s
-  `RenderBoundsChannel` — the one narrative home; this bullet is a pointer, not a
-  retelling. The one fact worth keeping local: every observation to date started at
-  `0s`, on the grid, so **nothing is known about whether the start floors too** —
-  assume neither.
+- **The custom-time-bounds field floors the render window to the millisecond; the
+  time selection does not.** Both observations and why only one bounds mode is
+  reachable: `render_settings.h`'s `kRenderBoundsTimeSelection` — the one narrative
+  home; this bullet is a pointer, not a retelling. Do not reintroduce
+  `RENDER_BOUNDSFLAG=0`.
 - `kRenderPreFaderStems` (&8192) is deliberately **not** used — REAPER offline
   render has no true pre-FX "dry" bit; FX scoping is done entirely by the
   FX-bypass-around-render mechanism, never by a render bit.