dasSpirv: base value/vector/matrix/swizzle emit machinery

[olegus8]

This is the first in a small stacked series that fills out the dasSpirv
daslang → SPIR-V emitter. It adds the general-purpose base machinery —
the scalar / vector / matrix / struct value core that the later layers
(resource & stage bindings; the fp16/int16/fp64 lattice lowering) build on.

What it adds

• Struct value types — a struct used as a local var (its own
  Function-storage OpVariable), as a function parameter and as a function
  result (OpFunctionParameter / return), undecorated; field reads lower to
  OpCompositeExtract (or through the local's pointer). A declared local
  zero-inits via OpConstantNull.
• Matrix constructors & products — floatCxR(col0..colN) builds a matrix
  from its columns in one OpCompositeConstruct; a column read m[i] is an
  OpCompositeExtract; and the three products stay distinct —
  OpMatrixTimesVector (m * v), OpVectorTimesMatrix (v * m),
  OpMatrixTimesMatrix (m * m). A module-scope const-column let matrix
  folds to OpConstantComposite.
• Write-swizzle lvalues — g.xyz = v lowers to load / OpVectorShuffle /
  store.
• Whole-vector equality — vec == vec / vec != vec reduced through
  OpAll / OpAny rather than a scalar compare.
• Bit-cast intrinsics — intBitsToFloat / floatBitsToInt &co →
  OpBitcast; mod / fmod → OpFMod; plus const arrays and vector
  index / shift.

Compiler fix (src/ast/ast_inline.cpp)

A bare (non-R2V) variable reference is an lvalue — a reference to
storage, and storage identity is stable for both a local and a global — so
it is inline-pure; only a global's value read (the R2V case) can change
mid-inline and is snapshotted (kept impure). Without this, a global array
being indexed inside an inlined call hoists into a local dynamic-array temp,
which the SPIR-V backend cannot represent.

Tests (tests/spirv)

Value structs & local structs, matrix constructors & products, write-swizzle,
whole-vector compare, bit-cast, fmod, const arrays, vector index / shift,
plus fail-closed fixtures (a non-constant matrix column index; a nested local
struct var). All are spirv-val-clean where spirv-val is available in CI.

Follow-ups in the stack

Resource & stage bindings (samplers, storage images, SSBOs, descriptor
arrays, subgroups, inline ray-query, workgroup size, fragment barycentrics,
mesh output), then the fp16 / int16 / fp64 lattice lowering.