Add MOM estimator options

[shenxiangzhuang]

Summary

• add explicit SPOT estimator, initial threshold, and excess update options
• keep SpotDetector::new defaulting to the existing Best/P2/>= behavior
• add FluxEV-style MOM support using sample variance, empirical thresholding, and strict > excess updates
• preserve serde compatibility for older SpotDetector JSON payloads

Validation

• cargo fmt --check
• cargo test --manifest-path crates/libspot-rs/Cargo.toml
• cargo clippy --manifest-path crates/libspot-rs/Cargo.toml --all-targets -- -D warnings

[github-actions]

▸ Cross-implementation results

commit 344786a · Linux · release builds

Consistency (basic example)

Metric	C	Rust FFI	Pure Rust
Anomalies	90007	90007	90007
Excess	7829	7829	7829
Normal	49902164	49902164	49902164
Z	6.237668	6.237668	6.237668
T	6.236165	6.236165	6.236165

Benchmark (hyperfine --warmup 1 --runs 5)

Command	Mean [s]	Min [s]	Max [s]	Relative
raw C	1.004 ± 0.001	1.003	1.006	1.00
Rust FFI (libspot)	1.520 ± 0.001	1.518	1.522	1.51 ± 0.00
Pure Rust (libspot-rs)	1.215 ± 0.004	1.209	1.220	1.21 ± 0.00

_{updated by Test cross-implementation consistency}