feat(compiler): add Enum.find_index/2 support for integer ranges

[yevbar]

Summary

Add Enum.find_index/2 compilation support for integer ranges (plain and stepped) to the Firebird Elixir-to-WASM compiler.

What it does

Compiles Enum.find_index/2 on integer ranges into efficient tail-recursive WASM loops with early termination:

# Plain range
Enum.find_index(1..n, fn i -> i > 5 end)

# Stepped range  
Enum.find_index(1..n//2, fn i -> i > 10 end)

# Negative step
Enum.find_index(10..1//-2, fn i -> i < 5 end)

Returns the 0-based index of the first element where the predicate is truthy, or -1 if no element matches (integer sentinel for nil in WASM).

Implementation

Generates a tail-recursive helper function:

__find_index_N__(i, stop, idx) =
  if i > stop do -1
  else if pred(i) do idx
  else __find_index_N__(i + step, stop, idx + 1)

This gets TCO-optimized into an efficient WASM loop. Follows the same pattern as the existing Enum.find/2, Enum.any?/2, and Enum.all?/2 implementations.

Supports

• Plain ranges (step=1)
• Positive stepped ranges
• Negative stepped ranges
• Dynamic step values (runtime sign check)

Tests

10 new end-to-end tests covering:

• Finding index of first matching element
• Returning -1 when no match
• Various predicates (even, divisible by 7, comparison)
• Stepped ranges (positive and negative)
• Edge cases (first element matches → index 0, empty range → -1)
• IR generation validity