Read a web page when he names one, and watch a few on a timer (#259)

The network fallback behind the local sources, off unless configured.

internal/crawl is pure: a stdlib robots.txt parser (group specificity,
wildcards, Crawl-delay, cached per host), HTML-to-plaintext extraction, and a
watcher that notes a watched page only when its text changed. It has no store
access and no net/http; cmd/mavend/crawls.go is the impure half.

Every limit is code and tested: the guarded fetcher from #258 enforces the host
allowlist/denylist, refuses private addresses in the dialer Control hook (so DNS
rebinding and each redirect hop are covered), caps size and redirects, times out,
and spaces requests per host. A robots.txt Disallow is refused with no override.

On demand, reading is a query source placed last in the chain, after his memory,
his notes, and the local Kiwix ZIMs once those are wired: no URL in the
utterance means no fetch, and only the URL ever leaves the box. Scheduled
watches write notes and announce nothing.

The vendored tree has no x/net/html, goquery or temoto/robotstxt, so the parsers
are stdlib. No new dependency.

cmd/mavend/voicewire.go

@@ -201,17 +201,20 @@ func wireVoice(cfg *config.Config, coreAPI ipc.CoreAPI, phr phraser.Phraser, mem
 
 	// ----- the handler (the reactive path; closes over stt / tts / router / coreAPI / memory) -----
 	h := &reactiveHandler{
-		stt:              transcriber,
-		tts:              synthesizer,
-		router:           rtr,
-		embedder:         emb,
-		api:              coreAPI,
-		tools:            exec,
-		matcher:          matcher,
-		replier:          replier,
-		phraser:          phr,
-		now:              time.Now,
-		feedsOn:          cfg.Feeds != nil,
+		stt:      transcriber,
+		tts:      synthesizer,
+		router:   rtr,
+		embedder: emb,
+		api:      coreAPI,
+		tools:    exec,
+		matcher:  matcher,
+		replier:  replier,
+		phraser:  phr,
+		now:      time.Now,
+		feedsOn:  cfg.Feeds != nil,
+		// nil unless `crawl.on_demand` is on: reading a page he names is a
+		// capability, and capabilities are off unless configured.
+		crawler:          onDemandCrawler(cfg),
 		weatherProvider:  weatherProvider,
 		weatherLocation:  weatherLocation,
 		memStore:         memStore,