refactor: eliminate 4 Rust backend duplications (retrieval lock, cosine sim, DiscoverMode, note context)

[WKJBryan]

Summary

Removes four distinct instances of duplicated logic in the Rust backend, all with no behavior change:

• run_retrieval helper — the 4-service read-lock pattern (search + graph + priority + retrieval) followed by retrieval.retrieve() was written out identically in commands/retrieval.rs, commands/memory.rs, and twice inline in commands/canvas.rs. Extracted to a single pub(crate) fn run_retrieval() in commands/mod.rs.

• fetch_note_contexts helper — two places in canvas.rs both locked the knowledge store, iterated retrieval results, truncated content to 1500 chars, and built (id, title, content) tuples. Extracted to a private helper. The twin path's context_notes filtering is preserved via a HashSet of found IDs.

• DiscoverMode enum — commands/zettelkasten.rs defined a private DiscoverMode with identical variants and parse() to services::link_discovery::DiscoverMode, then converted between them with to_service_mode(). Removed the local copy; import the service type directly (safe: IPC receives mode as Option<String>, no serialization boundary).

• cosine_similarity — standalone pub fn cosine_similarity(HashMap, HashMap) -> f64 in link_discovery.rs was identical math to SparseTfVector::cosine_similarity() in similarity.rs. Added pub fn sparse_cosine() to similarity.rs; link_discovery.rs imports and uses it.

Test plan

• All 178 Rust tests pass (cargo test)
• Clippy error count unchanged (34 pre-existing errors, 0 new)
• Lock precondition verified by code-reading: both canvas call sites use scoped {} blocks that drop all 4 locks before returning

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