Add range literals and indexed for sugar

[synapticvoid]

Summary

Add range iteration and indexed loop sugar to improve loop ergonomics.

Proposed Syntax

for i in 0..10 {}
for i in 0..10 by 2 {}
for elem, i in list {}

Semantics

• a..b is end-exclusive
• by sets step (default 1)
• for elem, i in list is indexed sugar over list + range index generation

Scope

• Parser/type-checker/runtime support for range literals and by
• Indexed loop sugar lowering/expansion rules
• Validation for invalid zero step and incompatible bounds/step types

Out of Scope

• Infinite ranges (0..)
• Reverse/descending shorthand beyond explicit negative step
• Multi-iterable zip (tracked separately)

Acceptance Criteria

• for i in 0..5 {} iterates 0,1,2,3,4
• for i in 0..10 by 2 {} iterates 0,2,4,6,8
• for elem, i in list {} binds element and index correctly