fix: switch to OnceLock, fix fmt/clippy, drop once_cell dep

- Replace once_cell::sync::Lazy with std::sync::OnceLock
- Fix cargo fmt formatting issues
- Fix clippy map_or -> is_some_and
- Remove once_cell from dev-dependencies

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

Cargo.lock

@@ -757,7 +757,6 @@ dependencies = [
  "libc",
  "log",
  "md5",
- "once_cell",
  "pest",
  "pest_derive",
  "predicates",

Cargo.toml

@@ -79,7 +79,6 @@ predicates = "3.0"
 tempfile = "3.8"
 serial_test = "3.0"
 uuid = { version = "1.6", features = ["v4"] }
-once_cell = "1.19"
 # Snapshot testing - captures full output and detects regressions
 insta = { version = "1.40", features = ["json", "redactions"] }
 # Property-based testing - exhaustive edge case coverage