test: make the ecosystem fault suite fail when the feature is deleted

Several assertions passed against code with the behaviour removed. The
independent-outage test shared no state to begin with, the capability
fixture used to prove read-only filtering was already mutating, and
route-level faults were simulated with a separate fake instead of the
shared one. The harness now takes per-route faults and a ticking clock,
so durations are measurable and one dead endpoint can be shown not to
mute a whole service. New cases cover a resolved reference with no
entity, a rejected credential, a malformed Praxis body, foreign items
in a scoped response, named truncation, traces staying out of facts,
and enrichment making progress while its oldest batch is backed off.

Found in review of #82.

cmd/mavend/fakeecosystem_test.go

@@ -27,11 +27,12 @@ type capturedRequest struct {
 type fakeServer struct {
 	*httptest.Server
 
-	mu       sync.Mutex
-	requests []capturedRequest
-	fault    int    // non-zero: every request gets this HTTP status instead of routing
-	garbage  string // non-empty: returned 200 verbatim instead of routing (malformed-contract lever)
-	delay    time.Duration
+	mu          sync.Mutex
+	requests    []capturedRequest
+	fault       int            // non-zero: every request gets this HTTP status instead of routing
+	routeFaults map[string]int // path prefix → status, for one endpoint failing alone
+	garbage     string         // non-empty: returned 200 verbatim instead of routing (malformed-contract lever)
+	delay       time.Duration
 }
 
 // newFakeServer starts a server dispatching to routes keyed by "METHOD
@@ -59,6 +60,14 @@ func newFakeServer(t *testing.T, routes map[string]http.HandlerFunc) *fakeServer
 			Header: r.Header.Clone(),
 		})
 		fault := fs.fault
+		if fault == 0 {
+			for prefix, status := range fs.routeFaults {
+				if hasPrefix(r.URL.Path, prefix) {
+					fault = status
+					break
+				}
+			}
+		}
 		garbage := fs.garbage
 		delay := fs.delay
 		fs.mu.Unlock()
@@ -114,6 +123,22 @@ func (fs *fakeServer) SetFault(status int) {
 	fs.fault = status
 }
 
+// SetRouteFault fails one endpoint while the rest of the server stays healthy,
+// which is the shape most real outages take: attention answers and pin is
+// down. Pass 0 to clear that route. A server-wide SetFault still wins.
+func (fs *fakeServer) SetRouteFault(pathPrefix string, status int) {
+	fs.mu.Lock()
+	defer fs.mu.Unlock()
+	if fs.routeFaults == nil {
+		fs.routeFaults = map[string]int{}
+	}
+	if status == 0 {
+		delete(fs.routeFaults, pathPrefix)
+		return
+	}
+	fs.routeFaults[pathPrefix] = status
+}
+
 // SetBody makes every subsequent request answer 200 with the given body,
 // bypassing the route table. Used to serve a malformed or contract-violating
 // payload where the transport itself is healthy. Pass "" to clear it.
@@ -198,6 +223,14 @@ func fixtureNexusResolvedFuture(entityID, displayName, entityType string) string
 	})
 }
 
+// fixtureNexusResolvedEmpty is the contract violation that decodes cleanly:
+// Nexus claims a resolve and delivers no entity. It must not read as "no such
+// entity", which would let the caller fall through to local execution with the
+// user's verb intact.
+func fixtureNexusResolvedEmpty() string {
+	return `{"status":"resolved"}`
+}
+
 func fixtureNexusNotFound() string {
 	return `{"status":"not_found"}`
 }
@@ -225,6 +258,16 @@ func fixtureHexisExecutionFailed(id, message string) string {
 	return mustJSON(map[string]any{"id": id, "status": "failed", "error": message})
 }
 
+// fixturePraxisAttentionScoped tags each item with an entity_id, which is what
+// a Praxis that understands the entity_id query parameter returns. A Praxis
+// that ignores it answers with untagged items from every entity.
+func fixturePraxisAttentionScoped(entityID string, items ...map[string]any) string {
+	for _, item := range items {
+		item["entity_id"] = entityID
+	}
+	return mustJSON(items)
+}
+
 func fixturePraxisAttentionItems(items ...map[string]any) string {
 	return mustJSON(items)
 }
@@ -243,18 +286,28 @@ func mustJSON(v any) string {
 // (e.g. asserting age-based digest ordering without sleeping).
 
 type fakeClock struct {
-	mu sync.Mutex
-	t  time.Time
+	mu   sync.Mutex
+	t    time.Time
+	step time.Duration // advanced on every read, so elapsed time is measurable
 }
 
 func newFakeClock(start time.Time) *fakeClock {
 	return &fakeClock{t: start}
 }
 
+// newTickingClock advances by step on every read. Durations measured across
+// hops are then non-zero without sleeping, which is what lets a test tell a
+// trace that measured something from one that measured nothing.
+func newTickingClock(start time.Time, step time.Duration) *fakeClock {
+	return &fakeClock{t: start, step: step}
+}
+
 func (c *fakeClock) Now() time.Time {
 	c.mu.Lock()
 	defer c.mu.Unlock()
-	return c.t
+	now := c.t
+	c.t = c.t.Add(c.step)
+	return now
 }
 
 func (c *fakeClock) Advance(d time.Duration) {