Refuse an image URL whose source path or query holds a # (closes #286)

A # written as %23 in the source path or query reached the parser
decoded and stayed in the source URL, where Go reads it as the start of
a fragment and drops it before fetching. So each text after the # was
another unsigned spelling of one upstream request, cached under its own
keys. An image URL whose decoded source path or query holds a # is now
refused with 400, signed or not, and README.md's rule for writing a
source path and query says a # cannot be part of either.

Model: opus-5-5

README.md

@@ -323,8 +323,9 @@ first `/`, the size and format after the last `/`, and between them the source
 path up to the first `?` and the source query after it. So the `?` before the
 query, any `?` in the query, and every `%` in the source path or query must be
 percent-encoded, as `%3F` and `%25`. Any other character may be written as
-itself or percent-encoded: `/`, `;`, `,`, `=` and `&` work either way. For
-example,
+itself or percent-encoded: `/`, `;`, `,`, `=` and `&` work either way. Neither
+the source path nor the source query may hold a `#`, and an image URL that
+writes one as `%23` is refused with 400. For example,
 `https://cdn.example.com/img?url=https://origin.example.com/my%20cat.jpg` at
 800x600 WebP is
 `/v1/image/cdn.example.com/img%3Furl=https://origin.example.com/my%2520cat.jpg/800x600.webp`,