feat(httpserver): HTTP_SERVER_START / _STOP with PRG handler dispatch

Adds hbrtl_ext/httpserver — a Five RTL extension that exposes a
single-process HTTP server controlled entirely from PRG.

Wire contract:
  HTTP_SERVER_START(cAddr, cHandlerFunc)  → blocking; returns NIL or cErr
  HTTP_SERVER_STOP()                      → graceful shutdown

PRG handler signature:
  FUNCTION OnRequest( hReq ) -> hResp
    hReq:  method, path, query, headers (hash), body, remote_addr
    hResp: status (default 200), headers (hash), body

Each request runs on its own hbrt.Thread via vm.NewThread(), the same
pattern pgserver uses for connection isolation. Handler panics are
caught and turned into a 500.

The package is wired into fnode's defaultRTL list so any build that
doesn't override --rtl picks it up automatically.

Verified end-to-end with app/echo_server.prg: GET/POST against :8089
return JSON envelopes with the correct method, path, query, body
length, remote_addr, and roundtripped user-agent header.

The mod_harbour-compatible AP_* surface (AP_METHOD, AP_RPUTS,
AP_JSONRESPONSE, etc.) will sit on top of this dispatcher in
sub-phase 1a.3 as PRG, not Go.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/fnode/main.go

@@ -35,6 +35,7 @@ const version = "fnode 0.1.0 — fivenode_go builder for Five"
 // Order is irrelevant — each package's init() is independent.
 var defaultRTL = []string{
 	"fivenode_go/hbrtl_ext/hello",
+	"fivenode_go/hbrtl_ext/httpserver",
 }
 
 func main() {