New record kind alongside h / s3 / ip / u: `p` reverse-proxies the
request to a full upstream URL while keeping the BNS host in the
address bar. Unlike `ip` (which pins the upstream to a raw IP + on-chain
TLS fingerprint), `p` uses the upstream's own DNS + public CA cert and
sends `Host:` of the upstream so vhost-based origins answer correctly.
Argus (record-picker.js):
- Placed AFTER `ip` in the apex chain: a name carrying both keeps its
existing pinned-IP behavior; names with only `p` get honored instead
of falling through to `u` and 302-ing away.
- Not applied to subdomain inheritance — `p` is single-URL by intent,
and inheriting through a subdomain would silently mangle the target.
Theseus (main.js serveBns):
- Fetches the upstream (path prefix from the `p` URL is preserved) and
returns the response body/status/content-type verbatim. Provenance
label "mirror" appears in the source badge.
Tests: record-picker.test.mjs covers `p`-only, `p`+ip precedence, and
the subdomain-suppression rule.
