router: add ActionCandidate type and ResolveActionCandidate

Introduce the typed boundary between routing and action resolution:

- ActionCandidate: Fn, Args, Source (route|matcher), Producer, Confidence
- ResolveActionCandidate(dec, m): standalone function usable by both
  the daemon and the eval harness
- Update eval harness Reach() to use ResolveActionCandidate instead of
  duplicating the matcher fallback logic

This is the routing-side half of the action-resolution boundary.
The daemon integration follows in the next commit.

internal/router/eval/reach.go

@@ -121,7 +121,7 @@ var PraxisAliases = map[string]string{
 //  1. A clarified act with a named entity target and no fn reaches Hexis before the clarify
 //     question is ever asked. That path runs on the raw slots, so the matcher
 //     does not get to fill fn first.
-//  2. Otherwise the act matcher may earn a fn from the text slot.
+//  2. Otherwise the act resolver produces an ActionCandidate from the route or matcher.
 //  3. A fn that is a Praxis capability alias dispatches to Praxis.
 //  4. An act with a named entity target reaches Hexis.
 //  5. Anything else stays inside Maven.
@@ -137,14 +137,9 @@ func Reach(d router.Decision, m router.ActMatcher) (Service, string) {
 		}
 		return ServiceNone, ""
 	}
-	fn, hasFn := d.Slots.Fn, d.Slots.HasFn
-	if !hasFn && d.Slots.Text != "" && m != nil {
-		if matched, _, ok := m.Match(d.Slots.Text); ok {
-			fn, hasFn = matched, true
-		}
-	}
-	if hasFn {
-		if capability, ok := PraxisAliases[strings.ToLower(strings.TrimSpace(fn))]; ok {
+	candidate := router.ResolveActionCandidate(d, m)
+	if candidate.ActionResolved() {
+		if capability, ok := PraxisAliases[strings.ToLower(strings.TrimSpace(candidate.Fn))]; ok {
 			return ServicePraxis, capability
 		}
 	}