refactor: split vault.rs into focused submodules
Break the 2338-line vault.rs into vault/ directory with 6 focused modules:
- mod.rs: core types (VaultEntry, Frontmatter), parse_md_file, scan_vault
- parsing.rs: text processing (snippet extraction, markdown stripping, date parsing)
- cache.rs: git-based incremental vault caching
- trash.rs: purge_trash with flat iterator chain
- rename.rs: note renaming with wikilink updates
- image.rs: attachment saving with filename sanitization
Also moves frontmatter update/delete wrappers to frontmatter.rs where they
belong, and converts public functions to accept &Path instead of &str.
CodeScene scores: mod.rs 10.0, parsing.rs 9.68, cache.rs 9.68,
trash.rs 9.38, rename.rs 9.68, image.rs 10.0 (all above 9.0 target).
All 169 Rust tests pass, 86.28% line coverage.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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