fix(producer): fall back to a temp dir when the font cache is unwritable

[miguel-heygen]

What

An unwritable font cache directory no longer aborts the render. It falls back to a temp
directory and warns once.

Closes #3412.

Why

The font cache holds Google Fonts downloads between runs. It is an optimisation — the
bytes are still fetchable without it — but failing to create it was fatal, because
fontCacheDir called mkdirSync unguarded during compile and the throw propagated out of
the render.

A first-time user on 0.8.8 lost their very first render to it:

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

Two things made it worse than the bug. The CLI's remediation was Try --docker for containerized rendering, which does not address an unwritable host directory. And
HYPERFRAMES_FONT_CACHE_DIR — which would have rescued them — appears in no user-facing
guide page.

How

fontCacheDir now catches the failure and falls back to one temp root per process, the same
shape the Lambda cache root a few lines above already uses:

fallbackFontCacheRoot ??= mkdtempSync(join(tmpdir(), "hyperframes-fonts-"));

A run still de-duplicates its own downloads; it just cannot reuse them next time. The
one-time warning names HYPERFRAMES_FONT_CACHE_DIR, so the escape hatch is discoverable at
the moment it is needed.

Test plan

End to end, before and after. Pointing HYPERFRAMES_FONT_CACHE_DIR at a path under a
chmod 500 parent, rendering a composition that requests Inter.

Before:

✗  Render failed
   EACCES: permission denied, mkdir '.../rocache/nested'
   Try --docker for containerized rendering

After:

[WARN] [Compiler] Font cache is not writable (EACCES: permission denied, mkdir '...').
       Caching to a temporary directory for this run instead; fonts will be re-downloaded
       next time. Set HYPERFRAMES_FONT_CACHE_DIR to a writable path to keep them.
  █████████████████████████  100%  Render complete

New regression test deterministicFonts-unwritableCache.test.ts drives
injectDeterministicFontFaces with the cache root under a read-only parent. Verified it
fails on the unfixed code rather than assuming:

EACCES: permission denied, mkdir '/var/folders/.../hf-font-locked-nSYU6R/nested'
(fail) unwritable font cache > still resolves fonts instead of aborting the render

All deterministicFonts suites green: 61 pass across 8 files.

Not covered

• The misleading Try --docker remediation is unchanged. It is emitted generically for
  render failures, so fixing it is a separate change about how that hint is chosen.
• HYPERFRAMES_FONT_CACHE_DIR is still absent from the docs site. Now surfaced in the
  warning, but a real docs page would be better.