Cache remark ownership lookup

[HassanAkbar]

related to #330

[Copilot]

Pull request overview

Improves performance and correctness of remark attachment by memoizing the node ownership lookup used when selecting the innermost candidate node for body comments, and ensuring memoized state is cleared after attachment (including on exceptions).

Changes:

• Memoize the node ownership map (owner_map) and reuse it across repeated innermost_candidate calls during a single attachment run.
• Update innermost_candidate to rely on the memoized ownership map rather than rebuilding from the passed node list.
• Add specs verifying the ownership map is built once and is cleared (along with the node index) if attach raises.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
spec/expressir/express/remark_attacher_spec.rb	Adds coverage for ownership-map memoization and cleanup-on-exception behavior.
lib/expressir/express/remark_attacher.rb	Introduces memoized owner_map, updates innermost_candidate to use it, and clears it in attach’s ensure block.

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