Support fractional age values for trial eligibility filtering

[imaurer]

Summary

• Change --age CLI flag and TrialSearchFilters.age field from u32 to f32 to support fractional year values (e.g. 0.5 for 6 months)
• Update verify_age_eligibility to drop the now-redundant age as f32 cast
• Add help text clarifying that decimals are accepted (e.g. 0.5 for 6 months)
• Add --age 0.5 --count-only example to the search trial EXAMPLES block and list trial reference output
• Add a fractional-age regression contract to spec/04-trial.md with timeout=180 to accommodate the CTGov age-only count traversal cost
• Add a CLI parse test asserting that non-numeric --age abc input is rejected with a clear error

Test plan

• biomcp search trial --age 0.5 --count-only exits 0 and returns a numeric Total:
• biomcp search trial --age 67 --count-only exits 0 (integer backward compatibility)
• biomcp search trial --age abc --count-only exits non-zero with a float parse error
• --help output shows "decimals accepted, e.g. 0.5 for 6 months" and the new EXAMPLES entry
• 596 unit tests pass, clippy clean