feat: add custom views backend — YAML parser, filter engine, Tauri commands
Introduce `.laputa/views/*.yml` for user-defined filtered note lists.
Rust backend: serde_yaml parsing, recursive AND/OR filter evaluation
with 10 operators (equals, contains, any_of, is_empty, before/after,
etc.), wikilink stem matching, and file CRUD. Three Tauri commands:
list_views, save_view_cmd, delete_view_cmd.
ADR 0040 documents the architecture.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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