perf: SymbolTableBuilder skips venv/git and supports incremental cache

[omsherikar]

Scope

refactron/analysis/symbol_table.py — SymbolTableBuilder.build_for_project

Problem

• python_files = list(project_root.rglob("*.py")) walks the entire tree with no excludes, so large trees (e.g. .venv, node_modules if misnamed, copied vendor trees) inflate work.
• When cache_dir is set and cache loads, the code returns the cached table and never refreshes; the TODO at line ~127 acknowledges missing incremental updates, so users either get stale symbols or pay full rebuild cost after deleting cache.

Suggested direction

• Reuse the same directory filtering pattern as RAGIndexer.index_repository (exclude .git, .venv, venv, __pycache__, etc.) or centralize "discover Python sources" in one helper.
• Incremental cache: track per-file hashes/mtimes in metadata; merge updated files into the in-memory SymbolTable instead of all-or-nothing reload.

Acceptance

• Medium/large fixtures with an artificial venv/ tree no longer dominate symbol build time.
• Documented behavior when cache is partial vs full rebuild.