Document how a source query goes into an image URL, and accept it (closes #255)

The query follows the source path after %3F. The image route now parses
the request's path decoded once instead of chi's wildcard, which holds
the path as the client wrote it whenever that is not Go's own encoding
of it, so a / ; or , written as %2F, %3B or %2C, as GenerateSignedURL
wrote them, hid the %3F and the request failed with 401. Now the ? and
every % in the source path or query must be percent-encoded and any
other character may be. GenerateSignedURL writes Go's own encoding of
the path. The README states the rule, with an example and its signature.

Model: opus-5-5

internal/imgcache/urlparser.go

@@ -61,9 +61,8 @@ type ParsedURL struct {
 	Format ImageFormat
 }
 
-// ParseImagePath parses the path captured by chi's wildcard:
-// <host>/<path>/<size>.<format>, or <host>/<path>/<size> for JPEG XL
-// This is the primary entry point when using chi routing.
+// ParseImagePath parses an image URL's path after /v1/image/, decoded once:
+// <host>/<path>/<size>.<format>, or <host>/<path>/<size> for JPEG XL.
 // Examples:
 //   - cdn.example.com/photos/cat.jpg/800x600.webp
 //   - cdn.example.com/photos/cat.jpg/0x0.jpeg
@@ -79,9 +78,10 @@ func ParseImagePath(path string) (*ParsedURL, error) {
 	return parseImageComponents(path)
 }
 
-// ParseImageURL parses a full URL path like /v1/image/<host>/<path>/<size>.<format>,
-// or /v1/image/<host>/<path>/<size> for JPEG XL
-// Use ParseImagePath instead when working with chi's wildcard capture.
+// ParseImageURL parses an image URL's path, decoded once as r.URL.Path holds
+// it: /v1/image/<host>/<path>/<size>.<format>, or /v1/image/<host>/<path>/<size>
+// for JPEG XL. A source query follows the source path after a "?", which the
+// URL writes as %3F.
 func ParseImageURL(urlPath string) (*ParsedURL, error) {
 	// Remove the /v1/image/ prefix
 	const prefix = "/v1/image/"