feat(lang): add for loop — range and list forms (V2 Tier 2c)

[particlesector]

Enables iterative geometry in SCAD scripts:

for (i = [0:4]) translate([i10, 0, 0]) sphere(r=3);
for (i = [0:2:8]) translate([i5, 0, 0]) cube([4,4,4]);
for (v = [1, 3, 7]) cylinder(h=v, r=2);

difference() {
cube([60, 12, 12], center=true);
for (i = [0:4]) translate([-24+i*12, 0, 0]) cylinder(h=20, r=3);
}

Changes:

• Token.h: For, Colon token kinds
• Lexer.cpp: "for" keyword, ':' operator
• AST.h: ForRange (isRange/start/step/end/list), ForNode; added to AstNode variant with makeFor() helper
• Parser.h/cpp: parseFor() — parses [start:end], [start:step:end], and [v0,v1,...] forms; disambiguates on first ':' vs ','
• Interpreter.h/cpp: getVar()/setVar() for loop variable binding
• CsgEvaluator.h/cpp: evalFor() — builds value sequence, binds loop var in interpreter, collects geometry from each iteration, unions all results; saves/restores outer variable binding; safety cap of 10000 iterations; empty range → nullptr
• Tests: 13 new tests across lexer, parser, and CSG evaluator (121 total, 688 assertions)
• tests/for_test.scad: 6 visual scenes including nested for loops