fix(engine,producer): drive forceScreenshot from one authoritative local

Miguel R4 blocker on #2359: my R3 fix at renderOrchestrator only updated
the observability copy, leaving the authoritative captureForceScreenshot
local at compileResult.forceScreenshot (false for auto→software). The
frameCapture side clamped its own local and correctly routed screenshot,
but downstream orchestrator code overwrote observability back to
beginframe from the still-false local at two sites:

  - Parallel-stream label at renderOrchestrator.ts:2293 mis-labelled the
    stream as 'beginframe' when actual capture was 'screenshot'.
  - capture_strategy telemetry at renderOrchestrator.ts:2440-2450
    overwrote the earlier observability correction, so the final
    captureMode observation flipped back to 'beginframe' while the
    engine actually captured screenshot.

Fix: extract the clamp into a caller-facing helper
applyConcreteGpuScreenshotClamp(current, resolvedGpuMode, cfg) that
returns the (possibly-promoted) new boolean. Callers assign it back to
their authoritative local, so routing + telemetry + strategy code read
one value.

Changes:

  - packages/engine/src/config.ts: new exported
    applyConcreteGpuScreenshotClamp; delegates to
    shouldClampToScreenshotForConcreteGpu but computes the caller's
    final value, not just the clamp decision. Reads the programmatic
    opt-out from cfg.forceScreenshotExplicitlyOptedOut. Idempotent on
    already-true input.
  - packages/engine/src/index.ts: export the new helper.
  - packages/engine/src/services/frameCapture.ts: replace the inline
    OR expression with applyConcreteGpuScreenshotClamp.
  - packages/producer/src/services/renderOrchestrator.ts: assign result
    into the AUTHORITATIVE captureForceScreenshot local (was updating
    only observability). Downstream parallel-stream label at :2293 and
    capture_strategy telemetry at :2440-2450 now read the corrected
    value.

Tests: 6 new caller-level cases for applyConcreteGpuScreenshotClamp
covering the exact matrix Miguel called out:

  - resolved software + default false → promotes to true (screenshot)
  - resolved software + programmatic opt-out → stays false (BeginFrame)
  - resolved hardware + default false → stays false
  - resolved software + already-true → stays true (idempotent)
  - resolved software + env PRODUCER_FORCE_SCREENSHOT=false → stays false
  - resolved software + undefined cfg → promotes to true (frameCapture path)

Local: 67/67 engine config tests pass (was 61). oxfmt clean.

packages/engine/src/services/frameCapture.ts

@@ -43,11 +43,7 @@ import {
   produceDrawElementFrameBatch,
 } from "./drawElementService.js";
 import { initThreeDProjection, detectCssEffectRisk } from "./threeDProjection.js";
-import {
-  DEFAULT_CONFIG,
-  shouldClampToScreenshotForConcreteGpu,
-  type EngineConfig,
-} from "../config.js";
+import { DEFAULT_CONFIG, applyConcreteGpuScreenshotClamp, type EngineConfig } from "../config.js";
 import type {
   CaptureOptions,
   CaptureVideoMetadataHint,
@@ -826,15 +822,15 @@ export async function createCaptureSession(
   // string, so `"auto"` that probes to software would otherwise slip through
   // and launch BeginFrame + SwiftShader (the exact combination the invariant
   // is meant to prevent). Both env and programmatic opt-outs preserved via
-  // the shared helper (the programmatic one carried on the config as
-  // `forceScreenshotExplicitlyOptedOut`, since at this point the boolean
-  // `forceScreenshot === false` is otherwise ambiguous between default and
-  // explicit opt-out).
-  const effectiveForceScreenshot =
-    forceScreenshot ||
-    shouldClampToScreenshotForConcreteGpu(resolvedGpuMode, forceScreenshot, process.env, {
-      programmaticOptOut: config?.forceScreenshotExplicitlyOptedOut ?? false,
-    });
+  // `applyConcreteGpuScreenshotClamp` (the programmatic one carried on the
+  // config as `forceScreenshotExplicitlyOptedOut`, since at this point the
+  // boolean `forceScreenshot === false` is otherwise ambiguous between
+  // default and explicit opt-out).
+  const effectiveForceScreenshot = applyConcreteGpuScreenshotClamp(
+    forceScreenshot,
+    resolvedGpuMode,
+    config,
+  );
   const preMode: CaptureMode =
     headlessShell &&
     isLinux &&