Run the world clock so day and night happen

set_time was declared nowhere and sent never, so the client's sky was frozen
wherever it started: the sun did not move, night did not fall, and nothing in
the world had a time.

26.1.2 replaced the old (gameTime, dayTime, doDaylightCycle) triple with a
registry of named clocks. The packet now carries a fixed-width game time plus a
map from world clock to (totalTicks, partialTick, rate); the client advances
each clock locally at its rate and drives the minecraft:day timeline -- a
keyframe track over a 24000-tick period -- from the overworld clock. The clock
registry is already among the 28 we sync, so the id is looked up rather than
hardcoded and the server refuses to start if it is missing.

The counter rides the entity tick loop because that is the only loop already
running at 20 TPS. It belongs on the single authoritative tick the engine still
needs; putting a sixth ticker beside the five that exist would make that worse.
Broadcast every second, which is what vanilla does -- the client interpolates in
between, so the resend only corrects drift.

The clock also persists now. The world metadata file was written once and never
touched again; it is atomically rewritable, carries gameTime and dayTime, and a
file written before those fields existed still opens and resumes at dawn as it
did. Saved every 30 seconds alongside the chunk autosave, and once more on
shutdown after the final flush.

internal/world/worldtime_test.go

@@ -0,0 +1,77 @@
+package world
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// TestWorldTimeRoundTrip checks the clock survives a restart. Without it the
+// sky snaps back to dawn every time the server comes up, however long the world
+// has been running.
+func TestWorldTimeRoundTrip(t *testing.T) {
+	dir := t.TempDir()
+	store, err := NewStoreForSeed(dir, 4242)
+	if err != nil {
+		t.Fatalf("open store: %v", err)
+	}
+	if gameTime, dayTime := store.WorldTime(); gameTime != 0 || dayTime != 0 {
+		t.Fatalf("a new world starts at %d/%d, want 0/0", gameTime, dayTime)
+	}
+	if err := store.SaveWorldTime(72_000, 18_000); err != nil {
+		t.Fatalf("save time: %v", err)
+	}
+	store.Close()
+
+	reopened, err := NewStoreForSeed(dir, 4242)
+	if err != nil {
+		t.Fatalf("reopen store: %v", err)
+	}
+	defer reopened.Close()
+	gameTime, dayTime := reopened.WorldTime()
+	if gameTime != 72_000 || dayTime != 18_000 {
+		t.Errorf("reopened at %d/%d, want 72000/18000", gameTime, dayTime)
+	}
+
+	// The seed guard has to survive the rewrite: it is the only thing stopping
+	// a world from being regenerated with different terrain.
+	if _, err := NewStoreForSeed(dir, 9999); err == nil {
+		t.Error("reopening with a different seed was accepted")
+	}
+}
+
+// TestWorldMetadataWithoutClock accepts a metadata file written before the
+// clock was persisted, rather than rejecting the world.
+func TestWorldMetadataWithoutClock(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(dir, "region"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	raw, err := json.Marshal(map[string]any{"format": 1, "seed": 7})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir, worldMetadataFile), raw, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	store, err := NewStoreForSeed(dir, 7)
+	if err != nil {
+		t.Fatalf("open a world written before the clock existed: %v", err)
+	}
+	defer store.Close()
+	if gameTime, dayTime := store.WorldTime(); gameTime != 0 || dayTime != 0 {
+		t.Errorf("clock read %d/%d from a file that has none, want 0/0", gameTime, dayTime)
+	}
+	if err := store.SaveWorldTime(10, 20); err != nil {
+		t.Fatalf("save time into an older world: %v", err)
+	}
+	reopened, err := NewStoreForSeed(dir, 7)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer reopened.Close()
+	if gameTime, dayTime := reopened.WorldTime(); gameTime != 10 || dayTime != 20 {
+		t.Errorf("after upgrading the file the clock reads %d/%d, want 10/20", gameTime, dayTime)
+	}
+}