fix: prevent Infinity anchorTimestamp ghost block spiral

The root cause of the compression spiral (101 failures, 2 hours):
resolveAnchorTimestamp returned Infinity when no message followed the
compression range. Infinity corrupted JSON serialization (became null),
and null timestamps in JS overlap checks coerced to 0, making every
range appear to overlap the ghost block b7.

Fixes:
- resolveAnchorTimestamp returns endTimestamp + 1 instead of Infinity
- Validate all timestamps are finite before creating a block (fail fast)
- Skip blocks with non-finite timestamps in overlap checks and compression
- Include existing block range in overlap error messages (diagnostic)
- Filter out corrupted blocks on session restore
- Guard synthetic message timestamp creation against non-finite values
- Add regression tests for Infinity anchor and null-timestamp blocks

pruner.test.ts

@@ -708,4 +708,115 @@ function findOrphanedToolUse(result: any[]): string | null {
   console.log("TEST 9 PASSED\n");
 }
 
+// ---------------------------------------------------------------------------
+// Test 10 — INFINITY ANCHOR BUG (regression test)
+//
+// Previously, when a compression block's range extended to the end of the
+// conversation, resolveAnchorTimestamp returned Infinity. This caused:
+//   1. JSON serialization turned Infinity into null, corrupting saved state
+//   2. Null timestamps in overlap checks caused false positives (every range
+//      appeared to overlap the ghost block)
+//   3. The model entered a compression spiral, unable to consolidate blocks
+//
+// Fix: resolveAnchorTimestamp returns endTimestamp + 1 instead of Infinity.
+// ---------------------------------------------------------------------------
+{
+  console.log("TEST 10: Infinity anchor timestamp regression");
+
+  // Conversation where the last message is at timestamp 4000.
+  // Compression block covers up to the end, so anchor should be 4001, not Infinity.
+  const messages: any[] = [
+    { role: "user",       content: [{ type: "text", text: "read file" }], timestamp: 1000 },
+    { role: "assistant",  content: [{ type: "toolCall", id: "toolu_1", name: "read", arguments: {} }], timestamp: 2000 },
+    { role: "toolResult", toolCallId: "toolu_1", toolName: "read", isError: false, content: [{ type: "text", text: "file data" }], timestamp: 3000 },
+    { role: "user",       content: [{ type: "text", text: "thanks" }], timestamp: 4000 },
+  ];
+
+  // Block that extends to the end of conversation
+  const state = makeState([
+    {
+      id: 1,
+      topic: "file read",
+      summary: "File was read.",
+      startTimestamp: 1000,
+      endTimestamp: 4000,
+      anchorTimestamp: 4001, // Fixed: was Infinity before the bugfix
+      active: true,
+      summaryTokenEstimate: 5,
+      createdAt: Date.now(),
+    },
+  ]);
+
+  const result = applyPruning(messages, state, makeConfig());
+
+  console.log("  Result messages:");
+  for (const m of result) {
+    const preview = Array.isArray(m.content)
+      ? m.content.map((b: any) => b.text ?? b.type ?? "?").join(" | ").slice(0, 60)
+      : String(m.content).slice(0, 60);
+    console.log(`    role="${m.role}"  ts=${m.timestamp}  content="${preview}"`);
+  }
+
+  // The synthetic message timestamp must be finite (not Infinity)
+  const synthetic = result.find(
+    (m: any) => m.role === "user" && typeof m.content?.[0]?.text === "string" && m.content[0].text.includes("Compressed section")
+  );
+  assert.ok(synthetic, "FAIL — no synthetic compressed message found");
+  assert.ok(
+    Number.isFinite(synthetic.timestamp),
+    `FAIL — synthetic message has non-finite timestamp: ${synthetic.timestamp}`
+  );
+  console.log(`  PASS: synthetic message has finite timestamp (${synthetic.timestamp})`);
+
+  console.log("TEST 10 PASSED\n");
+}
+
+// ---------------------------------------------------------------------------
+// Test 11 — CORRUPTED BLOCK WITH NULL/INFINITY TIMESTAMPS (resilience)
+//
+// Blocks from older sessions may have null/Infinity timestamps due to JSON
+// round-trip corruption. These blocks should be skipped during compression
+// application and should not block new compress operations.
+// ---------------------------------------------------------------------------
+{
+  console.log("TEST 11: corrupted block with null/Infinity timestamps is skipped");
+
+  const messages: any[] = [
+    { role: "user",       content: [{ type: "text", text: "hello" }], timestamp: 1000 },
+    { role: "assistant",  content: [{ type: "text", text: "hi" }], timestamp: 2000 },
+    { role: "user",       content: [{ type: "text", text: "bye" }], timestamp: 3000 },
+  ];
+
+  // Block with corrupted timestamps (null from JSON round-trip)
+  const state = makeState([
+    {
+      id: 1,
+      topic: "ghost block",
+      summary: "This block has corrupted timestamps.",
+      startTimestamp: null as any,  // null from JSON deserialization of Infinity
+      endTimestamp: null as any,
+      anchorTimestamp: null as any,
+      active: true,
+      summaryTokenEstimate: 5,
+      createdAt: Date.now(),
+    },
+  ]);
+
+  const result = applyPruning(messages, state, makeConfig());
+
+  console.log("  Result messages:");
+  for (const m of result) {
+    const preview = Array.isArray(m.content)
+      ? m.content.map((b: any) => b.text ?? b.type ?? "?").join(" | ").slice(0, 60)
+      : String(m.content).slice(0, 60);
+    console.log(`    role="${m.role}"  ts=${m.timestamp}  content="${preview}"`);
+  }
+
+  // All 3 original messages should survive (ghost block was skipped)
+  assert.strictEqual(result.length, 3, `FAIL — expected 3 messages, got ${result.length}`);
+  console.log("  PASS: corrupted block skipped, all original messages preserved");
+
+  console.log("TEST 11 PASSED\n");
+}
+
 console.log("All tests passed.");