Unwritable font cache directory aborts the render instead of degrading

[miguel-heygen]

Describe the bug

A render fails outright when the font cache directory cannot be created. The cache is an
optimisation — it stores Google Fonts downloads between runs — but failing to create it is
fatal, so a first render dies with a raw mkdir error.

Reported from the field on 0.8.8, on a first-ever render:

EPERM: operation not permitted, mkdir '<home>/.cache/hyperframes/fonts/inter'

The remediation the CLI offers for this failure is Try --docker for containerized rendering, which does not address an unwritable cache root.

Steps to reproduce

Platform-independent — any unwritable cache root reproduces it:

mkdir -p /tmp/rocache && chmod 500 /tmp/rocache
# a composition using any Google font (e.g. Inter)
HYPERFRAMES_FONT_CACHE_DIR=/tmp/rocache/nested npx hyperframes render --output renders/o.mp4

Actual behavior

✗  Render failed

   EACCES: permission denied, mkdir '/tmp/rocache/nested'
   Try --docker for containerized rendering

Same shape as the field report (EACCES vs EPERM are the same class here: the cache root
is not writable).

Expected behavior

An unwritable cache should degrade, not abort. The font can still be fetched per-run, or
cached to a temp directory. The render only needs the font bytes; where they are stored
between runs is an optimisation detail.

Root cause

packages/producer/src/services/deterministicFonts.ts:612:

function fontCacheDir(slug: string): string {
  const dir = join(resolveFontCacheRoot(), slug);
  if (!existsSync(dir)) {
    mkdirSync(dir, { recursive: true });
  }
  return dir;
}

The mkdirSync is unguarded, and this runs during compile, so the throw propagates and
fails the render.

Notes

• HYPERFRAMES_FONT_CACHE_DIR exists and would let an affected user redirect the cache, but
  it appears in no user-facing guide or reference page (grep -rlI HYPERFRAMES_FONT_CACHE_DIR docs matches only changelog / weekly-update / plan files). A
  user hitting this has no documented way out.
• Worth fixing the remediation hint at the same time: suggesting Docker for a permissions
  error on the host cache sends people the wrong way.