fix(codegen): check array percentrank indices

[luisleo526]

Summary

• reject negative, out-of-range, na, and non-finite array.percentrank indices for arrays with at least two elements
• preserve receiver-before-index one-evaluation order and the existing empty/singleton na behavior
• keep valid formula behavior unchanged and avoid internal-name collisions through nested generic-lambda bindings

This is intentionally bounds-only. PercentRank keyword routing, singleton semantics, return typing, and mixed-na behavior remain separate milestones.

Validation

• engine-enabled pytest: 1,696 passed / 2 skipped
• touched checked-array suites: 71/71
• native/Pyodide differential: 277/277 (263 accepted, 14 expected rejects)
• frozen source differential: 666/666 same status, exactly one changed generated file (the sole PercentRank consumer)
• sole consumer replay: 362 trades, candidate/base/accepted tape SHA-256 8f975cc05d67e55e99681263066b9d822f40a8b4cf7bddfa566a7d4ee22e182a
• representative get/set/remove generated output remains byte-identical to KI-48b
• independent review: no blockers
• git diff --check