bug(indexer): Node identity (name, path, line_number) is not unique; symbols merge and properties are clobbered #1393

tools/indexing/persistence/writer.py:409

merge_clause = f"MERGE (n:{label} {{name: row.name, path: $file_path, line_number: row.line_number}})"
# ...
UNWIND $batch AS row
{merge_clause}
SET n += row

Two distinct symbols in one file sharing a name and a line silently become one node, and SET n += row overwrites the first's args, class_context, end_line and cyclomatic_complexity with the last's.

This is the root cause of the 5530 vs 5501 discrepancy. Re-parsing the repo with the shipped parsers:

total parsed functions: 5530
distinct merge keys:    5501
lost:                     29   (22 colliding keys)

Two sources of collision:

• CSS — tools/languages/css.py:83 emits every selector as a Function. Grouped selectors (table, tbody, tr, td { }) produce several records with the same name and line.
• Minified/bundled JS — website/public/wasm/tree-sitter-core.js is one line, so genuinely different functions all carry line_number: 1. Node.endIndex and TreeCursor.endIndex collapse into a single node whose class_context is whichever was written last.

Fix. Add a disambiguator to the merge key (end_line, class_context, or a per-file ordinal). At minimum, detect key collisions in add_file_to_graph and log them.

_{Audit ID I-H05 · severity high · from an end-to-end audit of CGC 0.5.2 (63 findings). Reported with the reproduction as run.}