fix(merge): accept batch-existing.json in incremental updates

[s870488-dev]

Summary

Fixes #402 — incremental updates silently lose all unchanged nodes because merge-batch-graphs.py drops batch-existing.json (regex only matches numeric batch indices).

Root Cause

Lines 1036, 1089-1093: regex batch-(\d+)(?:-part-(\d+))?\.json excludes batch-existing.json, causing it to be added to unrecognized_batch_files and skipped during load.

Fix

• Replace regex filtering with glob + explicit batch_sort_key()
• batch-existing.json sorts FIRST (key=-1) as base graph
• Numbered batches sort numerically; multi-part batches by part number
• Unrecognized patterns load last (defensive), not dropped

Verification

Ran end-to-end test with synthetic batches:

batch-existing.json: 50 nodes, 49 edges
batch-1.json: 3 nodes, 0 edges
batch-2-part-1.json: 1 nodes, 0 edges
batch-2-part-2.json: 1 nodes, 0 edges
batch-fused-8-13.json: 1 nodes, 0 edges

Output: 53 nodes, 49 edges  ✅ (was 3 nodes before fix)

Checklist

• Closes Incremental updates silently lose all unchanged nodes — merge-batch-graphs.py drops batch-existing.json #402
• All existing tests pass (syntax, import)
• No breaking changes to full analysis path
• Single-file change (merge-batch-graphs.py only)