feat: session persistence — save and restore full state

[mpryor]

Summary

Save and restore full nless session state so users can pick up exactly where they left off.

Dependencies

Builds on top of:

• feat: hidden arrival timestamp metadata column for log ordering #29 — Arrival timestamp column (buffer state to persist)
• feat: bookmarks / saved views #26 — Saved views (views are a subset of session snapshots)
• feat: vi-style row marks for quick navigation #36 — Row marks (marks are session state to save/restore)

Details

• On exit (or via keybinding), save session state to disk
• Restore session on next launch with a flag (e.g., nless --restore or nless --session <name>)
• Session state includes:
  • All buffers and their data
  • Filters, sort order, column visibility/order per buffer
  • Active buffer selection
  • Scroll position and cursor location
  • Marks (feat: vi-style row marks for quick navigation #36), highlights (feat: multiple regex highlights with distinct colors #24), and alert rules (feat: alerting rules for streaming data #34)
  • Delimiter configuration per buffer

Use Cases

• Long-running log analysis sessions that span multiple sittings
• Sharing a specific analysis state with a colleague
• Recovering from accidental terminal closure
• Named sessions for different ongoing investigations

Considerations

• Storage format and location (e.g., ~/.config/nless/sessions/)
• Data size — storing full buffer data vs re-reading from source
• Stale data handling — warn if source file has changed since session was saved
• Auto-save on crash / unexpected exit
• Relationship with feat: bookmarks / saved views #26 (saved views) — views are reusable templates, sessions are full snapshots