fix(middleware): allow HTML rewrite for chunked and unknown-length bodies

Relax response-body gating so HTML and XHTML can be buffered when
Transfer-Encoding is chunked-only, or when Content-Length is missing,
while still rejecting non-identity encodings that are not chunked HTML
and other non-HTML cases.

Update modifyHTML to cap reads for unknown length, splice the original
stream back when the cap is hit, and document the behavior in the
package README. Extend tests for themed middleware and the rewrite gate.

internal/net/gphttp/middleware/README.md

@@ -14,7 +14,7 @@ This package implements a flexible HTTP middleware system for GoDoxy. Middleware
 - **Entrypoint Overlay Promotion**: Promote route-local middleware entries with `bypass` into matching entrypoint middleware for HTTP routes
 - **Dynamic Loading**: Load middleware definitions from files at runtime
 
-Response body rewriting is only applied to unencoded, text-like content types (for example `text/*`, JSON, YAML, XML). Response status and headers can always be modified.
+Response body rewriting is only applied to unencoded content. Known-size text-like responses (for example `text/*`, JSON, YAML, XML) are eligible, and HTML/XHTML responses may also be rewritten when their size is unknown or chunked, provided they stay within the rewrite buffer limit. Response status and headers can always be modified.
 
 Request-variable substitution reads request fields from the active outbound request. Upstream variables such as `$upstream_host` and `$upstream_url` resolve from the current route context, which is normally attached by the route / reverse-proxy layer before middleware executes.