Fix source_file path lookup for explain and affected

[behavio1]

Summary

This PR fixes full source_file path lookup for:

• graphify explain "<source-file-path>"
• graphify affected "<source-file-path>"

It also adds regression coverage for the case where a graph contains multiple
nodes from the same source file, such as a file-level node plus function nodes.

Root cause

Graphs can contain several nodes with the same source_file. For example, a
route file can have a file-level route.ts node and a GET() handler node from
the same path.

Before this change:

• explain could miss full path queries because _find_node() tokenizes the
  query, while the trigram prefilter only indexed the raw source_file.
• affected could fail to resolve a full path because exact source_file
  lookup returned multiple candidate nodes and no preferred seed.

What changed

• Include tokenized source_file text in the serve.py trigram search index.
• Let _find_node() match exact tokenized source_file paths.
• Prefer the file-level node when a full source path matches multiple nodes from
  the same file.
• Apply the same file-level preference in affected.resolve_seed().
• Add regression tests for explain, affected, and _find_node().

Validation

uv run python -m py_compile graphify/serve.py graphify/affected.py
uv run ruff check graphify/serve.py graphify/affected.py tests/test_serve.py tests/test_affected_cli.py tests/test_explain_cli.py
uv run pytest tests/test_serve.py tests/test_affected_cli.py tests/test_explain_cli.py -q
env -u GEMINI_API_KEY -u GOOGLE_API_KEY -u GOOGLE_GENAI_API_KEY -u MOONSHOT_API_KEY -u ANTHROPIC_API_KEY -u OLLAMA_BASE_URL uv run pytest tests/ -q

Full test result with provider env vars unset:

2430 passed, 28 skipped

[safishamsi]

Thanks @behavio1 — real bug, cleanly fixed, and the multi-node-per-source-file regression tests are exactly the right coverage. The file-node preference (L1 + basename match, with safe fallbacks) is sensible. Landed on v8 in 1b99496 with your authorship. Two small maintainer fixes on top: aligned trailing-separator handling between resolve_seed (affected) and _find_node (explain) — affected previously returned None for a trailing-slash path that explain resolved — and corrected the stale _find_node docstring; added regression tests for both. Verified end-to-end: path query resolves to the file node, label lookups unaffected, nonexistent path graceful, ambiguous no-L1 returns None. Closing as merged-by-port.