docs(plan): correct Rust support plan against current codebase state

[Eilodon]

• Task A4: replace the "executor must verify" guess about trait_item's class name field with a definitive fix, confirmed against tree-sitter-rust 0.23.3's real node-types.json — trait_item has field name, not type like impl_item, so class_name_field can't stay a single per-language constant; walk_symbols needs a per-node-kind branch.
• Task A3: correct pipeline.rs line citations (+21 lines throughout, from the same-file-candidate-preference fix that landed before this plan was committed) and note why, so line numbers match HEAD.
• Task B5: correct the ci-server/src/lib.rs insertion point description to match the actual spawn_blocking block structure.
• Task B6: rename to the existing bN_name/run_benchmark.py convention and wire it into the already-reserved "B2" row in benchmarks/README.md instead of a new ad hoc top-level folder.
• Task A1: note that the shipped fix stays string-based (no nested use-group support), since the research doc's phrasing overstated scope.
• Minor: fix a stale function name reference in Task A5.

Claude-Session: https://claude.ai/code/session_013HgQtsfovjqekyVw7ZjCS9