RichLog widget for append-only log panes

[meszmate]

Summary

Adds RichLog, a virtualized log widget for long-running apps that need a persistent log/audit pane (think k9s events, build output, server logs).

Features

• Bounded ring buffer — oldest entries drop on overflow
• Per-entry severity (trace/debug/info/warn/err) with default color theme
• Timestamps (HH:MM:SS) — opt-in via `show_timestamps`
• Renders only the visible window — O(rows) regardless of buffer size
• Follow-mode (auto-scroll to tail) toggled off when user scrolls back
• `setMinLevel` filters out entries below a threshold
• `setSearch` filters and highlights matching substrings
• Standard scroll keybindings: arrow keys, page up/down, home/end, j/k/g/G

Why a new widget

Distinct from existing components:

• Viewport — raw text, no structure or filtering
• Toast — transient notifications, not persistent
• Markdown / CodeView — content viewers, not append targets

Test plan

• `zig build test` — added 6 tests covering capacity ring, level filter, search filter, follow-mode auto-scroll, scrollUp disabling follow, and view height bound