Centralize Severity string parsing across bindings

[dean0x]

Locations: mds-napi:687, mds-wasm:429, mds-python:794

The serde_json::from_str(&format!("\"{s}\"")) severity-parse hack is triplicated across three bindings.

Deferred From: PR #171 (Closes #61) — mds lint release gate
Review: .devflow/docs/reviews/feat-mds-lint-61/2026-07-11_1046/resolution-summary.md

Reason for Deferral:
Centralization as impl FromStr for mds::Severity in mds-core requires coordinated change across 4 crates. Best tackled as focused refactor post-release.

Acceptance Criteria:

• Implement impl FromStr for Severity in mds-core/src/lint/diagnostic.rs
• Remove hack from crates/mds-napi/src/lib.rs:687
• Remove hack from crates/mds-wasm/src/lib.rs:429
• Remove hack from crates/mds-python/src/lib.rs:794
• All binding tests pass