fix(types,sa,codegen): signed idiv, multi-decl init, pointer arithmetic

[rieske]

Summary

Fixes three correctness bugs found by mutation fuzzing of the master language surface:

1. Signed division/modulo – idiv was preceded by xor rdx,rdx instead of cqo, so negative dividends could SIGFPE or yield wrong quotients/remainders.
2. Multi-declarator initializers – all initializers were analyzed before any name was inserted, so forms like int a = 1, b = a / int a = 1, *p = &a falsely reported undefined symbols.
3. Pointer arithmetic – p+i, p-i, p-q, and pointer ++/-- did not scale by pointee size (byte math instead of element math).

Design

• Type::isPrimitive() excludes indirection so int* is not treated as a scalar.
• Shared type::classifyPointerArithmetic (TypeQuery) for SA result typing and CG IR selection.
• New IR: PointerOffset / PointerDiff; shared backend scaledBaseIndex with array indexing.
• Integer Add/Sub stay unscaled; pointer ++/-- use a documented byte step on Inc/Dec.
• Declarations: visit each declarator, insert, then analyze initializer (C visibility).

Test plan

• Functional: signed div/mod (locals + global), multi-declarator cross-init, pointer scale/diff/++, char ptrdiff
• Unit: Type.pointerToSignedInteger not primitive; TypeQuery.classifyPointerArithmeticForms
• Array indexing still green (shared scaledBaseIndex)
• CI full make test / ctest

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[coveralls]

coverage: 93.503% (+0.2%) from 93.287% — fix/fuzz-signed-div-pointer-arith into master