fix: generate DDSketch quantile-ratio candidates without evidence

[zzylol]

Closes #446

Before this PR

quantile_over_time(0.9, data[5m]) / quantile_over_time(0.5, data[5m]) fell back to KeepPreAsap without input-domain evidence. After exposing the candidate, target-aware search removed it before the backend could inspect it.

After this PR

• The default strategy generates the direct DDSketch ratio candidate without domain evidence.
• A target-aware PlanSpace retains the candidate with guarantee: None for ASAPQuery-backend to inspect and decide using its own evidence. Other unknown or target-missing candidates remain filtered.
• Planner automatic global_selection does not choose the uncertified ratio; materialization falls back to exact KeepPreAsap. A ratio certified by complete valid evidence remains automatically selectable.
• Invalid supplied domains are rejected, including when the other operand has no evidence.
• The Planner/backend ownership boundary is documented in the library and ratio design docs.

Verification

• New regression failed before the auto-selection fix and passes after it.
• cargo fmt --all -- --check
• cargo test -p asap-aware-mapping
• cargo test -p asap-integration-tests
• cargo clippy -p asap-aware-mapping -p asap-integration-tests --all-targets -- -D warnings