feat(lang): Tier C — children(), echo(), assert(), recursive functions

[particlesector]

Implements the four Tier C language completeness items:

• children() / $children: modules can now receive and render the child geometry passed at the call site. children() yields all children, children(i) yields the i-th. $children holds the count. A stack-based context correctly handles nested children() across module layers.

• echo(): captures all arguments into CsgScene.echoMessages; MeshBuilder forwards them to the diagnostics panel as Info-level entries (cyan).

• assert(cond[, msg]): records an Error diagnostic in CsgScene.evalDiags when the condition is falsy; MeshBuilder merges these into BuildResult.

• Recursive functions: already functional from Tier A; confirmed with a new CsgEvaluator test driving geometry from fib().

Supporting changes:

• parsePrimary now handles SpecialVar tokens as VarRef so $children, $fn, $fs, $fa are readable in expression context.
• $fn/$fs/$fa are seeded into the interpreter env at evaluate() entry.
• DiagnosticsPanel renders Info level in cyan to distinguish echo output from warnings (yellow) and errors (red).
• 22 new unit tests in test_csg_evaluator.cpp + tier_c_test.scad.