fix: signal-backed marker reactivity and stale signal re-push

Two issues discovered during testing:

1. data-effect expressions for signal-backed markers short-circuited
   before reading $signalID when the map hadn't loaded yet. Datastar
   never tracked those signals as dependencies, so the effect never
   re-ran. Fix: read signal values unconditionally before the guard.

2. prepareSignalsForPatch never reset changed=false on regular signals
   (only computed ones), causing every SyncSignals() call to re-push
   all signals — including stale server-side values for the draggable
   pin, overwriting the client-side drag position.

context.go

@@ -291,6 +291,7 @@ func (c *Context) prepareSignalsForPatch() map[string]any {
 			}
 			if sig.changed {
 				updatedSigs[sigID.(string)] = fmt.Sprintf("%v", sig.val)
+				sig.changed = false
 			}
 		case *computedSignal:
 			sig.recompute()