Accept only an origin written exactly as a browser sends it (closes #61)

access_control_allow_origin is now "*", or http or https, a host that is
an IP address as net/netip writes it (IPv6 in brackets) or a lowercase
host name whose last part contains a letter, and an optional port 1-65535
with no leading zero that is not the scheme's default. The value must
equal the origin rebuilt from those parts; anything else aborts startup
naming the key, its variable and the value. README.md and
config.example.yml say an origin is scheme, host and optional port,
exactly as the browser sends it.

Model: opus-5-5

TODO.md

@@ -38,9 +38,9 @@ exhaustion
   server's write timeout and the per-request timeout); each has a
   `PIXA_` variable; durations are positive Go duration strings, the size a
   whole number of bytes up to 1 GiB, the origin `*` or one scheme, host
-  name or IP address and optional port; an invalid value aborts startup
-  naming the key and the value; documented in `config.example.yml` and
-  `README.md`.
+  and optional port, exactly as the browser sends it; an invalid value
+  aborts startup naming the key and the value; documented in
+  `config.example.yml` and `README.md`.
 - 2026-09-28 strip metadata from processed images (closes #82): every output is
   exported with govips' `StripMetadata`, so it carries no EXIF, XMP, IPTC or ICC
   profile; the image is first turned upright with `AutoRotate` (before sizes are