PolyBench CST-path -> line-range resolver for fragment metrics

[nikolay-e]

Context

PolyBench-500 ships CST-node paths (modified_nodes) not line ranges; the adapter never resolves them, so PolyBench contributes file-level metrics only. Fragment/line metrics on 500 multi-language instances are being left on the table.

Tasks

• Resolver: CST path -> (start_line, end_line) via tree-sitter pass on the post-patch worktree
• Handle path-mismatch/parse-failure rows explicitly (status, not silent drop)
• Backfill fragment/line metrics for PolyBench in the next sweep (pairs with 15)

Acceptance

• =95% of modified_nodes resolve; PolyBench fragment-level rows appear in artifacts