feat(cli): surface extract-cache dir in doctor + add --frames-cache-dir sugar

Windows users with the OS temp dir on a small system drive have hit
C: exhaustion mid-render (Slack ts=1784219488 · CLI v0.7.58 · win32
15 GB / 8-core, ~5500 frames). The engine already honors
HYPERFRAMES_EXTRACT_CACHE_DIR for relocation, but the knob was
undocumented and invisible in diagnostics — the reporter had to piece
together a 4-flag compound workaround including EXTRACT_CACHE_DIR=off.

Changes:
- Extract the env-var resolver into a public engine API
  (resolveExtractCacheDir, defaultExtractCacheDir,
  EXTRACT_CACHE_DIR_DISABLED_ALIASES) with a typed resolution shape
  distinguishing "disabled by user" vs "default" vs "env override".
- Add a Frames-cache check to `hyperframes doctor` that reports the
  effective directory, its free space, source (env or default), and
  fails with a relocation hint when <2 GB free at that mount.
- Add `hyperframes render --frames-cache-dir <path>` as discoverable
  CLI sugar for the env var, including the opt-out aliases
  (off/none/false/0) and CWD-safe absolute-path resolution.
- Document the flag in docs/packages/cli.mdx with the field-signal
  citation, and add a render example row for the Windows workflow.
- Cover both surfaces with unit tests (6 doctor cases + 4 engine
  cases including all disabled-alias variants).

Refs Slack #hyperframes-cli-feedback ts=1784219488 (win32 v0.7.58).

Co-authored-by: Via <via-heygen[bot]@users.noreply.github.com>

packages/engine/src/index.ts

@@ -52,7 +52,11 @@ export {
   scaleProtocolTimeoutForComposition,
   shouldClampToScreenshotForConcreteGpu,
   applyConcreteGpuScreenshotClamp,
+  resolveExtractCacheDir,
+  defaultExtractCacheDir,
+  EXTRACT_CACHE_DIR_DISABLED_ALIASES,
   type EngineConfig,
+  type ExtractCacheDirResolution,
 } from "./config.js";
 export {
   DEFAULT_VP9_CPU_USED,