v0.1.3: structured HTTP 413 handling for the wdgwars.pl upload cap

LOCOSP rolled out a temporary 15 MB body cap on every wdgwars.pl upload
endpoint on 2026-06-05. Workaround for CloudLinux LVE killing the PHP
worker mid-buffer on bodies above roughly 20 MB. The server now returns
a structured 413 envelope with max_bytes + received instead of a
generic 500. Cap is expected to be removed in roughly two weeks when
LOCOSP completes a host migration.

Before this release, send_chunk would log 413 as a generic 4xx
rejection. Functionally safe (no retry loop, no API blast), but
structurally noisy: callers had no signal to differentiate "shrink your
batch" from "your key is wrong". This release adds that signal.

Added

• 413 + payload-too-large envelope detection in send_chunk. When
  the server returns the cap envelope, gungnir logs max_bytes,
  received, and the body size, records a cooldown via
  gungnir.cooldown.record (default 30s, or retry_after if the
  server populates it), and returns (1, envelope) so the caller can
  see the structured fields. No retry, no BatchAborted: the caller may
  have other queued payloads to attempt with smaller bodies.
• 3 new tests in tests/test_transport.py covering: envelope detection
  • cooldown recording + single-attempt contract; server-supplied
    retry_after honored over the 30s default; non-LOCOSP 413 (CF or
    upstream HTML body) falling through to the generic rejected branch.

Notes for callers

The HMAC envelope is an atomic signed blob, so gungnir cannot bisect
the payload on its own without invalidating the signature. Callers
that produce variable-size payloads (Muninn aircraft batches,
Heimdall mesh-node bursts) should treat a 413 as "shrink the batch
and call again". For most feeders this never triggers: aircraft and
mesh snapshots are kilobytes per cycle.