Honest completion of roadmap #8. Previously a NON-429 media failure (a
connection reset, a timeout, a truncated stream, a 5xx) was an immediate
terminal "error" for the pass — only retried on the NEXT walk. Now
_fetch_to_file retries TRANSIENT failures in-place with backoff (transport
blips incl. mid-download, 429 honoring Retry-After, and 5xx; up to 3 tries),
while PERMANENT failures (404 gone / 403 forbidden) fail fast straight to the
error → dead-letter path — re-fetching them is pointless. This makes the
transient-vs-permanent split explicit instead of leaning on the next-tick
cycle. (#1's 429 backoff + #7's dead-letter covered most of #8's value; this
is the missing in-pass transient piece I'd loosely marked "folded".)
Tests: a connection blip / a 5xx is retried then succeeds; a 404 errors with
NO retry; an exhausted transient becomes a terminal error.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
bvandeusen
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