safefetch: an allowlist, and no proxy to go round it

Two follow-up findings on the same file, both real.

**A proxy would have made the whole check pointless.** http.DefaultTransport
carries ProxyFromEnvironment, so with HTTP_PROXY set the connection goes to the
proxy and the proxy connects to whatever the URL named -- the address being
checked would be the proxy's, and the one that matters would never be looked at.
The safe client now sets no proxy, and a test asserts it.

**"Not RFC 1918" is not the same as "public".** The check is now an allowlist:
an ordinary global unicast address that is not in an IANA special-purpose range.
The ranges that were being let through include 100.64.0.0/10, which is
carrier-grade NAT and routes to a real host on a real network, and 64:ff9b::/96
and 2002::/16, which are NAT64 and 6to4 and carry an IPv4 address inside them --
127.0.0.1 included. None is private by Go's definition and none is somewhere a
podcast is served from.

Checked against the live library before committing: 130 real URLs -- all 44
feeds and two enclosures from every show -- resolve to addresses this permits,
so nothing Peter subscribes to stops mirroring. That check is kept as a test
which skips where the state file is not present, because the failure mode of
being too strict is a feed that silently stops updating.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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