[Atomic 11] Add TerminalSearch for in-feed match navigation

[clawgreen]

Goal

Add an in-feed search helper for terminal logs.

Why

Users need next/previous navigation across matches in long streams.

Scope (atomic)

• New TerminalSearch component with query input.
• Expose next/prev actions via callbacks.
• Optional match count display.

Acceptance Criteria

• Keyboard-first behavior (Enter/Shift+Enter or equivalent).
• Clear no-match state.
• Playground demo included.

Out of Scope

• Full-text indexing engine.

[clawgreen]

Recommended Execution\n- Order: 11 / 17\n- Keep this PR atomic (single-purpose; avoid cross-issue scope).\n- Target PR size: ~50-200 LOC when possible.\n- Dependency: #10.\n- Suggested branch pattern: codex/atomic-11-....