Support custom alignment in rawAlloc for refc GC

[Copilot]

Small chunks allocate memory in fixed-size cells positioned at exact multiples of cell size, preventing arbitrary alignment. This adds alignment support by forcing big chunk allocation for alignments > MemAlign.

Changes:

• Added alignOffset: uint16 to BigChunk to track alignment adjustment from data start
• Extended rawAlloc/rawAlloc0 with alignment and offset parameters (defaults preserve existing behavior)
• Implemented applyAlignment() to compute aligned cell pointer: aligns (basePtr + offset) then backs up by offset
• Updated gc.nim to pass typ.base.align when allocating ref objects
• Fixed pointer arithmetic in allObjects, deallocBigChunk, isAllocatedPtr, getCell, and getObjSizes to account for alignOffset
• Added tests for 16/32/64/128-byte alignments including nested structures

Example:

type
  MyType128 = object
    a {.align(128).}: int

let x = new MyType128  # Allocated with 128-byte alignment
doAssert cast[int](x) mod 128 == 0

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