docs(latest): correct the browser-site comments the comix change made stale

Review findings from #98: BrowserFetcher's doc still described two sites and
two read shapes, Get's doc enumerated them a second time, and the Fallback
field doc omitted comix. The comix fixture comment now records the live
in-tab body it was re-checked against.

backend/internal/latest/sites.go

@@ -47,9 +47,9 @@ type browserRead struct {
 	// Done reports whether the payload arrived.
 	Done func(body string) bool
 	// Fallback allows the plain-TLS fetcher when no browser is configured.
-	// False skips the Site instead. kagane is false — a plain fetch would
-	// only ever retrieve a challenge page — and novelfull is true, because
-	// its challenge is a live time-varying fact (AGENTS.md).
+	// False skips the Site instead. kagane and comix are false — a plain fetch
+	// would only ever retrieve a challenge page — and novelfull is true,
+	// because its challenge is a live time-varying fact (AGENTS.md).
 	Fallback bool
 }