Support align pragma with generic type parameters

[elijahr]

This PR extends the deferred pragma infrastructure from #25403 to support the align pragma with generic type parameters, as well as a new capability: type-level alignment.

Summary

1. Type-level align pragma: The align pragma is now valid on type definitions, not just fields and variables
2. Field-level deferred align: Field alignment can use expressions like alignof(T)
3. Deferred evaluation: Align expressions are evaluated during generic instantiation

Example

const CacheLine = 64

type
  CacheAligned[T] {.align: (if alignof(T) > CacheLine: alignof(T) else: CacheLine).} = object
    value: T

  Aligned128 {.align: 128.} = object
    data: array[64, byte]

# alignof(int) < CacheLine, so CacheLine (64) is used
var x: CacheAligned[int]
x.value = 42

# alignof(Aligned128) > CacheLine, so alignof(T) (128) is used
var y: CacheAligned[Aligned128]
y.value.data[0] = 1

static:
  doAssert alignof(CacheAligned[int]) == CacheLine
  doAssert alignof(CacheAligned[Aligned128]) == 128

Implementation

Pragma processing (pragmas.nim):

• wAlign added to typePragmas set (previously only in fieldPragmas/varPragmas)
• Modified wAlign handler to support both type-level and field-level deferred expressions
• Power-of-2 validation for alignment values

Instantiation (semtypinst.nim):

• Added wAlign case to applyDeferredPragma for type-level alignment
• New applyDeferredFieldPragma: Evaluates field-level deferred pragmas
• New evaluateDeferredFieldPragmas: Orchestrates field pragma evaluation
• Integration with replaceTypeVarsN for field processing

Tests

• tests/pragmas/tdeferred_align.nim - Align pragma at type and field level

Dependencies

• Requires Support size pragma with generic type parameters #25403 for deferred pragma infrastructure