fix(codegen): route array percentrank keywords

[luisleo526]

Summary

• register the index parameter for array.percentrank keyword merging
• support namespace all-keyword (including reversed keyword order), namespace mixed, global identifier method, and arbitrary-expression method forms
• preserve receiver-before-index one-evaluation behavior through the existing checked PercentRank lowering

This does not broaden PercentRank semantics. Array-typed UDF parameter method dispatch is a separate pre-existing receiver-routing residual; its positional form is also currently unsupported.

Validation

• engine-enabled pytest: 1,701 passed / 2 skipped
• focused keyword/native runtime: 6/6
• native/Pyodide differential: 277/277 (263 accepted, 14 expected rejects)
• positional frozen manifest: 666/666 byte-identical, 660 OK / same 6 errors
• sole positional consumer generated C++ therefore remains byte-identical; no trade replay change is possible
• independent review: approved, no scoped blockers
• git diff --check