Add type stubs for landmarkdiff package

[dreamlessx]

Description

Add py.typed marker and ensure all public APIs have complete type annotations for better IDE support and mypy compatibility.

Current State

Most modules have type hints, but some are incomplete or use Any where more specific types could be used.

Proposed Changes

1. Add py.typed marker file to landmarkdiff/
2. Audit all public functions for missing type annotations
3. Replace Any with concrete types where possible
4. Add mypy to CI with --strict mode for the public API surface
5. Fix any revealed type errors

Key Files

• landmarkdiff/inference.py — pipeline public API
• landmarkdiff/evaluation.py — metrics functions
• landmarkdiff/api_client.py — client public API
• landmarkdiff/postprocess.py — post-processing pipeline

Acceptance Criteria

• py.typed marker present
• mypy --strict landmarkdiff/ passes (or errors are explicitly ignored with comments)
• All public function signatures have return type annotations