feat(engine): add opt-in image timeout and scene-paint hedging

IMAGE_TIMEOUT_MS sets a per-attempt hard deadline (AbortSignal.timeout);
IMAGE_HEDGE_MS races a second identical scene-paint request when the
first is still pending past the threshold. Both default to OFF when
unset, preserving historical behavior for self-hosted deploys.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AGENTS.md

@@ -138,6 +138,7 @@ Use `.env.example` as the source of truth. Never commit `.env.local`, API keys,
 
 - Text and Vision use `TEXT_*` and `VISION_*`; default protocol is `openai_compatible`, with native `anthropic` and `google` available via `TEXT_PROVIDER` / `VISION_PROVIDER`.
 - Image uses `IMAGE_*`; supported protocols are `runware`, `openai_compatible`, native `openai`, and native `google`. When `IMAGE_PROVIDER` is unset, Runware is inferred from `*.runware.ai` URLs and otherwise falls back to OpenAI-compatible image generations.
+- `IMAGE_TIMEOUT_MS` (per-attempt hard deadline) and `IMAGE_HEDGE_MS` (Painter scene-paint hedging: race a second request when the first is still pending after the threshold) are both OFF when unset — the default path must stay byte-identical to historical behavior. Hedging applies only to the Tier-A scene paint, never to portraits, and never fires after a fast failure (saturation guard). Client-side engine configs (`resolveEngineConfig`) intentionally do not set these fields.
 - TTS supports Xiaomi MiMo (voicedesign + voiceclone) or StepFun (preset voices auto-selected by keyword scoring), inferred from `TTS_BASE_URL` (host containing `stepfun.com` → StepFun, otherwise → MiMo). `CharacterVoice` is a discriminated union on `provider`; synth dispatches on the voice's own tag so a session may carry both shapes through a provider switch. Blank config means silent mode.
 - `MOCK_IMAGE=true` skips image generation and returns a placeholder for cheap local iteration.
 - `NEXT_PUBLIC_IMAGE_PROXY_URL` and `NEXT_PUBLIC_IMAGE_PROXY_ALLOWED_HOSTS` opt into browser-side image proxying for allowed hosts.