[GrantFox][Medium] Reduce feed rendering cost with bounded virtualization

[greatest0fallt1me]

Summary

Reduce feed rendering cost with bounded virtualization

Why this matters

Large market feeds can cause slow interaction and excessive memory use.

Scope

Add bounded virtualization or incremental rendering while preserving keyboard navigation and URL state.

Acceptance criteria

• Initial interaction meets a documented budget.
• Rows remain accessible and stable while scrolling.
• Pagination and filters retain correct results.
• Performance tests prevent regression.

Validation

Add regression coverage for existing behavior, failure modes, authorization boundaries, and compatibility. The implementation must pass the repository CI checks.

Non-goals

• Typo-only, formatting-only, or documentation-only changes.
• Unrelated refactors or dependency upgrades.
• Weakening existing security, authorization, CI, or production safeguards.

Contributor application

Before implementation, comment with relevant experience, a 1–4 bullet approach, and an estimate for opening the first draft PR. Wait for maintainer assignment before coding.

PR requirements

Use a feature branch, include Closes #<issue-number>, check every acceptance criterion, link criteria to code/tests, explain security and failure-mode considerations, and pass CI.

Reward-readiness

This is a substantive GrantFox campaign issue. Merge and CI success do not by themselves guarantee reward eligibility; final reward-readiness is determined by campaign review.