Route simple descendant chains through scoped lookups

[jdalton]

Changes

• Route simple descendant chains through scoped lookups when the estimated cost is lower.
• Keep callbacks, element scopes, fragments, and legacy hosts on the normal route.
• Preserve string-like selector argument conversion.
• Keep route estimates separate from query results so mutations cannot return cached answers.

Validation

• 554 Node tests passed; existing expected failures and browser skips remain.
• 55 browser harness tests passed.
• Type checking, lint, and formatting passed.
• Combined coverage passed unchanged thresholds: 84.82% statements, 75.92% branches, 83.49% functions, and 83.62% lines.

Performance comparisons will use the updated benchmark harness.