refactor(search): document+guard visibility filter precondition; drop redundant dev-dep

- Remove serde_json from [dev-dependencies] (already in [dependencies])
- Add debug_assert! in search_objects visibility filter as defense-in-depth
- Extend search_objects doc-comment with visibility precondition
- Clarify estimated_total_hits.unwrap_or(0) is safe under offset/limit pagination
- Add brief comment on with_crop_length(20) explaining ~20-word context window

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/search/Cargo.toml

@@ -16,6 +16,5 @@ serde_json.workspace = true
 [dev-dependencies]
 tokio.workspace = true
 uuid.workspace = true
-serde_json.workspace = true
 sqlx.workspace = true
 domain = { path = "../domain" }