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

index.ts

@@ -81,7 +81,16 @@ export default function (pi: ExtensionAPI) {
         const data = entry.data as any
 
         if (data?.compressionBlocks) {
-          state.compressionBlocks = data.compressionBlocks
+          // Filter out blocks with corrupted (null/NaN/Infinity) timestamps —
+          // these were caused by Infinity anchorTimestamp values that became
+          // null after JSON round-trip.
+          const validBlocks = data.compressionBlocks.filter(
+            (b: any) =>
+              Number.isFinite(b.startTimestamp) &&
+              Number.isFinite(b.endTimestamp) &&
+              Number.isFinite(b.anchorTimestamp),
+          )
+          state.compressionBlocks = validBlocks
           state.nextBlockId = data.nextBlockId ?? state.compressionBlocks.length
           state.tokensSaved = data.tokensSaved ?? 0
           state.totalPruneCount = data.totalPruneCount ?? 0