Clarify trial filter help: facility modes, phase 1/2 semantics, sex all no-op

[imaurer]

Summary

Improves the clarity of biomcp search trial help text for three filter flags that had confusing or missing guidance:

• --facility: Now distinguishes between text-search mode (cheap, plain string match) and geo-verify mode (materially more expensive, triggers N extra fetches per result) that activates when --lat/--lon/--distance are also provided.
• --phase 1/2: Now explains that the value 1/2 matches the ClinicalTrials.gov combined Phase 1/Phase 2 label (studies tagged as both phases simultaneously), not Phase 1 OR Phase 2.
• --sex all: Now explains that all (also any/both) resolves to no sex restriction — no sex filter is sent to ClinicalTrials.gov — so users know the flag is a no-op rather than a missing echo.

No changes to underlying filter logic, parsers, search, or render behavior.

Changes

• src/cli/mod.rs: Updated doc comments for --facility, --phase, and --sex; added render_trial_search_long_help() helper and three CLI help contract tests.
• spec/04-trial.md: Added "Trial Help Explains Special Filter Semantics" spec section asserting all three clarified surfaces.

Test plan

• cargo test — 619 passed, 0 failed
• cargo clippy --all-targets --all-features — clean
• cargo fmt --check — clean
• make spec — 103 passed (pre-existing Gene→Articles network timeout in spec/02-gene.md is unrelated)
• biomcp search trial --help — all three clarifications visible in rendered output