[Atomic 17] Add virtualization option to TerminalFeed for large log volumes

[clawgreen]

Goal

Scale feed rendering for very large streams.

Why

Without virtualization, large log buffers can degrade performance.

Scope (atomic)

• Add optional virtualization mode to TerminalFeed.
• Preserve API parity with non-virtualized mode.
• Document tradeoffs and limits.

Acceptance Criteria

• Large feeds remain responsive.
• Scrolling and jump-to-latest still function.
• Clear fallback behavior when virtualization is disabled.

Out of Scope

• Rewriting all existing feed components.

[clawgreen]

Recommended Execution\n- Order: 17 / 17\n- Keep this PR atomic (single-purpose; avoid cross-issue scope).\n- Target PR size: ~50-200 LOC when possible.\n- Dependency: #16.\n- Suggested branch pattern: codex/atomic-17-....