[Atomic 09] Extend TerminalLog to support structured entries (non-breaking)

[clawgreen]

Goal

Upgrade TerminalLog to support structured feed data without breaking existing lines: string[] usage.

Why

Backward compatibility plus richer data unlocks stronger feed UX.

Scope (atomic)

• Add optional structured prop (e.g., entries) while keeping lines functional.
• Internally reuse TerminalLogLine where possible.
• Keep max-lines and auto-scroll behavior.

Acceptance Criteria

• Existing string-based consumers continue to work unchanged.
• Structured entries render with level metadata.
• API docs/examples updated.

Out of Scope

• Filtering/search UI.

[clawgreen]

Recommended Execution\n- Order: 9 / 17\n- Keep this PR atomic (single-purpose; avoid cross-issue scope).\n- Target PR size: ~50-200 LOC when possible.\n- Dependency: #8.\n- Suggested branch pattern: codex/atomic-9-....