test(kbblock): cover fade RenderFaded/Tick/Refresh/seed paths

Phase 4b shipped Seed + RenderFaded + Tick + Refresh against the
sdkfade primitives but the test file still only covered the original
no-fade behavior. These add:

  - Add() populates Seed
  - RenderFaded == Render when d <= N (safe period)
  - RenderFaded masks underscores once d crosses the safe period
  - Tick drops entries past the M% threshold + leaves fresh ones
  - Refresh resets LastRefreshAtTurn + regenerates Seed

internal/kbblock/kbblock_test.go

@@ -5,6 +5,8 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+
+	sdkfade "git.hangman-lab.top/hzhang/Plexum-sdk-go/fade"
 )
 
 func TestSessionDir(t *testing.T) {
@@ -126,6 +128,73 @@ func TestRenderEmpty(t *testing.T) {
 	}
 }
 
+func TestAddPopulatesSeed(t *testing.T) {
+	b, _ := Open(t.TempDir(), "a", "s")
+	e := b.Add(1, "K", "t", "x", 0)
+	if e == nil || e.Seed == 0 {
+		t.Errorf("Seed should be non-zero, got %d", e.Seed)
+	}
+}
+
+func TestRenderFadedDistanceZeroIsIdentity(t *testing.T) {
+	b, _ := Open(t.TempDir(), "a", "s")
+	b.Add(1, "K", "t", "hello world", 5)
+	plain := b.Render()
+	faded := b.RenderFaded(5, sdkfade.DefaultFadeParams())
+	if plain != faded {
+		t.Errorf("d=0 should be no-op:\nplain=%q\nfaded=%q", plain, faded)
+	}
+}
+
+func TestRenderFadedAppliesMaskOverTime(t *testing.T) {
+	b, _ := Open(t.TempDir(), "a", "s")
+	b.Add(1, "K", "t", "the quick brown fox jumps over the lazy dog", 0)
+	plain := b.Render()
+	faded := b.RenderFaded(20, sdkfade.DefaultFadeParams())
+	if plain == faded {
+		t.Errorf("d=20 should mask, but identical: %q", plain)
+	}
+	if !strings.Contains(faded, "_") {
+		t.Errorf("expected underscore masking in faded output: %q", faded)
+	}
+}
+
+func TestTickDropsHighlyFaded(t *testing.T) {
+	b, _ := Open(t.TempDir(), "a", "s")
+	b.Add(1, "K", "t", "short", 0)
+	b.Add(2, "K", "t", "this entry stays freshish", 100)
+	res := b.Tick(105, sdkfade.DefaultFadeParams())
+	hasID1Dropped := false
+	for _, id := range res.FadedOut {
+		if id == 1 {
+			hasID1Dropped = true
+		}
+	}
+	if !hasID1Dropped {
+		t.Errorf("entry 1 (d=105) should have crossed drop threshold: faded=%v", res.FadedOut)
+	}
+	if !b.Has(2) {
+		t.Error("entry 2 (d=5) should still be present")
+	}
+}
+
+func TestRefreshResetsClock(t *testing.T) {
+	b, _ := Open(t.TempDir(), "a", "s")
+	b.Add(1, "K", "t", "content", 0)
+	oldSeed := b.Lookup(1).Seed
+	got := b.Refresh([]int{1, 99}, 50)
+	if len(got) != 1 || got[0] != 1 {
+		t.Errorf("Refresh returned %v, want [1]", got)
+	}
+	e := b.Lookup(1)
+	if e.LastRefreshAtTurn != 50 {
+		t.Errorf("LastRefreshAtTurn = %d, want 50", e.LastRefreshAtTurn)
+	}
+	if e.Seed == oldSeed {
+		t.Error("Refresh should regenerate Seed")
+	}
+}
+
 func TestSaveEmptyNoFile(t *testing.T) {
 	dir := t.TempDir()
 	b, _ := Open(dir, "a", "s")