feat(producer): renderStretch to re-time short compositions across longer scenes (#2676)

Linear: VA-1859

## Problem

For a `fit_to_scene` B-roll where the composition's intrinsic timeline (e.g. `data-duration=1.0s` → 30 frames) is shorter than the scene it fills (e.g. 4.8s narration), the producer renders only the intrinsic 30 frames and the downstream compositor frame-holds/PTS-stretches that fixed clip to the scene length. Spreading 30 unique frames over 4.8s starves motion to ~6 effective fps → a visibly choppy result. Root cause: the producer welds one `composition.duration` to both the frame count and the 1:1 seek mapping, with no notion of a target output length.

## Fix

Add optional `renderStretch: number` (default `1.0` = no-op), `renderStretch = intrinsic / target`:

- **Frame count** comes from the target: `outputDuration = intrinsic / renderStretch`, `totalFrames = outputDuration × fps` (`probeStage.ts`). `composition.duration` stays intrinsic (drives video/audio windows).
- **Per-frame seek** is scaled: `time = (frameIndex / fps) × renderStretch`, so the N output frames map across `[0, intrinsic]` — a fresh frame per output frame.

All seek sites go through a single shared `outputFrameToTimelineSeconds(frameIndex, fps, renderStretch)` helper (`core.types.ts`), consumed by every capture path so none can silently diverge:
- parallel (`parallelCoordinator.ts`), `sdr_streaming` (`captureStreamingStage.ts` ×3), `sdr_disk` (`captureStage.ts`), HDR loops.
- DrawElement + static self-verify (`frameCapture.ts`) — ground-truth seek uses the same mapping, so PSNR compares like-for-like (no spurious verification failure on stretched comps).
- Distributed path: `renderStretch` threaded through `DistributedRenderConfig` → chunk workers, and **folded into the plan hash only when `!= 1`** so a pre-stretch cached plan is never reused.

With `renderStretch = 1` (or omitted → `?? 1`): every seek is `×1.0` (IEEE-754 identity), frame counts unchanged, and the plan hash is byte-identical — a provable no-op. `player.ts` absolute-seek is untouched.

## Verify

- typecheck (core + engine + producer): pass. lint/format/fallow/commitlint: pass. `planHash` + `renderRequest` unit suites: pass.
- Adversarial self-review found + fixed three capture-path gaps (streaming, self-verify, distributed) before this revision.
- **Not yet runtime-verified** on a real render — needs a fit_to_scene render at `renderStretch < 1` confirming N distinct frames over the target length (draft until then).

Paired with experiment-framework#42766, which computes and forwards `renderStretch = hf intrinsic / scene duration`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

packages/engine/src/services/parallelCoordinator.ts

@@ -23,6 +23,7 @@ import {
   type CapturePerfSummary,
   type BeforeCaptureHook,
 } from "./frameCapture.js";
+import { outputFrameToTimelineSeconds } from "@hyperframes/core";
 import { DEFAULT_CONFIG, type EngineConfig } from "../config.js";
 import { assertSwiftShader } from "../utils/assertSwiftShader.js";
 import { readWebGlVendorInfoFromCanvas } from "../utils/readWebGlVendorInfoFromCanvas.js";
@@ -368,6 +369,10 @@ async function captureFrameRange(
   let framesCaptured = 0;
   const outputOffset = task.outputFrameOffset ?? 0;
   const stride = task.frameStride ?? 1;
+  // Per-frame seek ×renderStretch maps frames across [0, intrinsic] (1 = no-op).
+  const renderStretch = captureOptions.renderStretch ?? 1;
+  const seekTime = (i: number): number =>
+    outputFrameToTimelineSeconds(i, captureOptions.fps, renderStretch);
   // Depth-2 pipelined drawElement produce (HF_DE_PARALLEL_STREAM spike): frame
   // k's in-page worker encode overlaps frame k+stride's produce phase — the
   // same shape as the sequential worker-encode loop. Only engaged when the
@@ -387,7 +392,7 @@ async function captureFrameRange(
     let prev: { idx: number; encodeResult: Promise<Buffer> } | null = null;
     for (let i = task.startFrame; i < task.endFrame; i += stride) {
       if (signal?.aborted) throw new Error("Parallel worker cancelled");
-      const time = (i * captureOptions.fps.den) / captureOptions.fps.num;
+      const time = seekTime(i);
       if (dbg && i < task.startFrame + dbgWin) {
         console.log(`[par:w${task.workerId}] +${Date.now() - dbgT0}ms produce ${i} start`);
       }
@@ -431,7 +436,7 @@ async function captureFrameRange(
   }
   for (let i = task.startFrame; i < task.endFrame; i += stride) {
     if (signal?.aborted) throw new Error("Parallel worker cancelled");
-    const time = (i * captureOptions.fps.den) / captureOptions.fps.num;
+    const time = seekTime(i);
     const fileFrameIdx = i - outputOffset;
 
     if (onFrameBuffer) {