Symbol graph: O(n²) colocation edges, duplicate accumulation, brace counting ignores strings

[haasonsaas]

Found during deep code review of symbol_graph.rs

1. O(n²) colocation edges per file (lines 179-192)

For N symbols per file, creates N*(N-1) bidirectional edges. 100-symbol module = 9,900 edges.

2. BFS traversal is O(E² log E) (lines 256-303)

Sorts entire frontier each iteration instead of using BinaryHeap. Nodes re-processed multiple times.

3. Duplicate edges accumulate without dedup

add_edge appends without checking for existing same-pair edges.

4. find_block_end ignores strings/comments (lines 489-511)

Brace counting treats braces inside string literals as real. let s = "}}}}" decrements depth by 4.

Acceptance

• Cap colocation edges or skip large-symbol files
• Use BinaryHeap for frontier
• Deduplicate edges on insert
• String/comment-aware brace counting

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