test(sequencer): add timing tests for CheckpointProposalJob

Adds timing tests for the CheckpointProposalJob that verify timing
budgets and blocks built using a new `ManualDateProvider` so we mock
advancing time.

Tests the proposal job and timetable in coordination using mocked time
for determinism.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: spalladino

yarn-project/foundation/src/timer/date.ts

@@ -31,4 +31,46 @@ export class TestDateProvider extends DateProvider {
     this.offset = timeMs - Date.now();
     this.logger.warn(`Time set to ${new Date(timeMs).toISOString()}`, { offset: this.offset, timeMs });
   }
+
+  /** Advances the time by the given number of seconds. */
+  public advanceTime(seconds: number) {
+    this.offset += seconds * 1000;
+  }
+}
+
+/**
+ * A date provider for tests that only advances time via explicit advanceTime() calls.
+ * Unlike TestDateProvider, this does NOT track real time progression - time is completely
+ * frozen until explicitly advanced. This eliminates flakiness from tests taking
+ * varying amounts of real time to execute.
+ */
+export class ManualDateProvider extends DateProvider {
+  private currentTimeMs: number;
+
+  /**
+   * @param initialTimeMs - Initial time in milliseconds. Defaults to a round timestamp for easy visualization.
+   */
+  constructor(initialTimeMs: number = Date.UTC(2025, 0, 1, 0, 0, 0)) {
+    super();
+    this.currentTimeMs = initialTimeMs;
+  }
+
+  public override now(): number {
+    return this.currentTimeMs;
+  }
+
+  /** Sets the current time to the given timestamp in milliseconds. */
+  public setTime(timeMs: number) {
+    this.currentTimeMs = timeMs;
+  }
+
+  /** Advances the time by the given number of seconds. */
+  public advanceTime(seconds: number) {
+    this.currentTimeMs += seconds * 1000;
+  }
+
+  /** Advances the time by the given number of milliseconds. */
+  public advanceTimeMs(ms: number) {
+    this.currentTimeMs += ms;
+  }
 }