[Atomic 10] Add TerminalFilterBar for level/source/text filtering

[clawgreen]

Goal

Introduce a compact filter bar for terminal feed views.

Why

As feeds grow, users need fast narrowing by severity/source/text.

Scope (atomic)

• New component with typed props + JSDoc.
• Level toggles + text input.
• Optional source selector.
• Playground integration with sample log data.

Acceptance Criteria

• Keyboard accessible controls.
• Component is controlled via props/callbacks.
• Works with existing theme tokens.

Out of Scope

• Server-side filtering.

[clawgreen]

Recommended Execution\n- Order: 10 / 17\n- Keep this PR atomic (single-purpose; avoid cross-issue scope).\n- Target PR size: ~50-200 LOC when possible.\n- Dependency: #9.\n- Suggested branch pattern: codex/atomic-10-....