Expose Leiden tuning flags

[fizyxbt]

Fixes #30.

Summary:

• add --leiden-max-iter, --leiden-epsilon, --leiden-min-iter, and --leiden-skip-refinement to grafly analyze
• apply custom values as overrides on top of the fast or thorough preset
• document the new flags in the README
• add CLI parsing coverage for custom values and invalid non-positive values

Tests:

• cargo fmt --all -- --check
• cargo test -p grafly-cli
• cargo run -p grafly-cli -- analyze --help | rg "leiden-(thorough|max-iter|epsilon|min-iter|skip-refinement)"