feat(plugin): add session restore lifecycle support

[fcoury]

Why

Plugins can react to editor events today, but they do not have a deterministic way to persist workspace state at shutdown or restore it during startup. That makes session-style plugins impossible without baking the behavior directly into the editor. This change adds the missing lifecycle and state APIs, then uses them to ship a built-in session_restore plugin.

What Changed

• Added editor session snapshots for file-backed buffers, active buffer, cursor and viewport positions, cwd, and window split layout.
• Added plugin APIs for red.getEditorState(), red.restoreEditorState(snapshot), and namespaced JSON red.storage.
• Added startup/shutdown plugin extension points: async-safe activate(red), editor:ready, awaited beforeExit(red, state), and context-aware deactivate(red).
• Added plugins/session_restore.js, enabled it in default_config.toml, and documented the new APIs in docs/PLUGIN_SYSTEM.md and types/red.d.ts.

How to Test

1. Enable session_restore = "session_restore.js" under [plugins] in a Red config.
2. Start Red without explicit file arguments, open a couple of real files, move the cursors, and create a split.
3. Quit cleanly, then start Red again from the same cwd without explicit file arguments.
4. Confirm that the same file-backed buffers, active window, cursor positions, viewport positions, and split layout are restored.
5. Start Red with an explicit file argument and confirm session restore is skipped for that launch, so the requested file opens normally.
6. Delete or rename one file from a saved session, restart from the same cwd, and confirm the remaining files restore while the missing file is skipped and logged.

Targeted tests:

• cargo test snapshot_round_trips_split_layout
• cargo test
• timeout 3 target/debug/red CLAUDE.md after cargo build to verify startup reaches the editor instead of failing during plugin activation