fix(engine,producer): apply software-GPU screenshot invariant at concrete-resolved point

Addresses Miguel's R1 blockers:

1. `browserGpuMode: "auto"` that runtime-probes to software slipped past the
   `resolveConfig` clamp — that clamp only sees the pre-resolve string. Add
   `shouldClampToScreenshotForConcreteGpu(resolvedGpuMode, currentForceScreenshot, env)`
   in `packages/engine/src/config.ts` and apply it at BOTH concrete-resolution
   sites:
   - `packages/engine/src/services/frameCapture.ts`: downgrades `preMode`
     from "beginframe" to "screenshot" when resolved GPU is software (respects
     `PRODUCER_FORCE_SCREENSHOT=false` env opt-out), fixing the routing.
   - `packages/producer/src/services/renderOrchestrator.ts`: updates
     `captureObservability.forceScreenshot` (and thus `captureMode`) at the
     same call site, fixing the observability truth on the auto → software
     case.

2. New unit tests in `config.test.ts`:
   - Documents the auto-branch gap (resolveConfig leaves auto as
     forceScreenshot=false — the runtime companion closes it).
   - 5 branch tests on `shouldClampToScreenshotForConcreteGpu` covering
     software / hardware / already-forced / env-opt-out / non-"false" env
     values.
   Full suite: 56/56 pass.

Scope narrowing on Blocker 2: the distributed rendering path at
`packages/producer/src/services/distributed/plan.ts:753-754` and
`renderChunk.ts:462-466` explicitly hardcodes `browserGpuMode:"software",
forceScreenshot:false` post-resolveConfig and stays outside this PR's
invariant boundary. `compileStage` may still flip it to true for alpha
formats, but generic MP4 distributed renders on SwiftShader hosts remain
BeginFrame. That's a separate architectural cleanup (needs its own
behavior-change trace); the PR body now scopes the invariant to the
in-process CLI/orchestrator path.

packages/engine/src/index.ts

@@ -48,6 +48,7 @@ export {
   resolveConfig,
   DEFAULT_CONFIG,
   scaleProtocolTimeoutForComposition,
+  shouldClampToScreenshotForConcreteGpu,
   type EngineConfig,
 } from "./config.js";
 export {