refactor(quartz): swap FsEventStore clock ctor param for protected now()

The clock injection existed only for NIP-40 expiration tests. A public
constructor parameter that ~every caller ignores is API clutter, so
move it to a subclass seam:

- FsEventStore is now `open class`; no more `clock: () -> Long` param.
- `protected open fun now(): Long = TimeUtils.now()` is the override
  point. Production always takes the default; tests subclass.
- FsExpirationTest gains a private `ClockedStore(root, source)` that
  overrides `now()`. Semantics unchanged, all 113 fs tests still green.

Public `FsEventStore` ctor is now `(root, indexingStrategy, relay)` —
no behavioural-drift surface that callers have to learn.

quartz/src/jvmTest/kotlin/com/vitorpamplona/quartz/nip01Core/store/fs/FsExpirationTest.kt

@@ -40,13 +40,24 @@ class FsExpirationTest {
     private lateinit var root: Path
     private var clockNow: Long = 1_000_000
 
-    private lateinit var store: FsEventStore
+    /**
+     * Test subclass overriding the `now()` seam so we can drive NIP-40
+     * expiration at exact timestamps — no wall-clock sleeps, no flakes.
+     */
+    private class ClockedStore(
+        root: Path,
+        private val source: () -> Long,
+    ) : FsEventStore(root) {
+        override fun now(): Long = source()
+    }
+
+    private lateinit var store: ClockedStore
 
     @BeforeTest
     fun setup() {
         Secp256k1Instance
         root = Files.createTempDirectory("fs-exp-")
-        store = FsEventStore(root, clock = { clockNow })
+        store = ClockedStore(root) { clockNow }
     }
 
     @AfterTest