Add struct types to Halide

[alexreinking]

This PR adds struct types to Halide. This allows for inputs and outputs to be defined in terms of popular wire formats, even if they aren't prepared for efficient computation. For example, you can define GGML's q5_0 and q8_0 formats like so:

Type q5_0 = Type::Struct({{"d", Float(16)}, {"qh", UInt(32)}, {"qs", UInt(8), 16}});
Type q8_0 = Type::Struct({{"d", Float(16)}, {"qs", Int(8), 32}});

Then you can write a pipeline that consumes them:

ImageParam x{q4_0_type(), 1, "x"};
ImageParam y{q8_0_type(), 1, "y"};

Var b("b"), k("k"), u("u");
RDom r(0, x.dim(0).extent(), 0, 32, "r");  // block, quant

// Dequantize one 4-bit weight to float: quant k is a nibble of the
// packed byte k % 16 -- its low nibble for k < 16, its high nibble
// otherwise -- biased by -8 to signed [-8, 7], times the block delta.
Expr nib = field(x(b), "qs")[k % 16];  // uint8
Func x_wt("x_wt");
x_wt(k, b) = cast<float>(field(x(b), "d")) * 
             (cast<int32_t>(select(k < 16, nib % 16, nib / 16)) - 8);

// Dequantize one int8 activation to float: quant k is int8 k, times the
// block delta.
Func y_wt("y_wt");
y_wt(k, b) = cast<float>(field(y(b), "d")) * 
             cast<int32_t>(field(y(b), "qs")[k]);

// Dequantize each side and sum every product.
Func qdot{"qdot"};
qdot() = 0.0f;
qdot() += x_wt(r[1], r[0]) * y_wt(r[1], r[0]);

As you can see, struct types support both individual elements, as well as fixed-size array elements. Fields are accessed with a named field(expr, "name") intrinsic. The tests include additional cases for struct-typed fields and output structs.

This adjusts the Type and halide_type_t representations in the following ways:

1. Type: like Handle, a Struct type has a compile-time known field layout.
2. halide_type_t: structs get their own kind, and the reserved field records the number of bytes in the struct.

All layouts are expected to be packed. No padding is inserted around or in between field elements.

Breaking changes

None—it's a new feature.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[codecov]

Codecov Report

❌ Patch coverage is 18.25558% with 403 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (alexreinking/rfactor-hoisting@3c6ea1b). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/LowerStructTypes.cpp	20.49%	118 Missing and 10 partials ⚠️
src/IROperator.cpp	5.38%	123 Missing ⚠️
src/Type.cpp	25.00%	69 Missing and 12 partials ⚠️
src/ConstantBounds.cpp	0.00%	15 Missing ⚠️
src/Type.h	48.00%	9 Missing and 4 partials ⚠️
src/IRPrinter.cpp	0.00%	10 Missing and 2 partials ⚠️
src/IROperator.h	0.00%	7 Missing ⚠️
src/CodeGen_Vulkan_Dev.cpp	0.00%	4 Missing ⚠️
src/runtime/HalideRuntime.h	0.00%	1 Missing and 2 partials ⚠️
src/CodeGen_C.cpp	77.77%	0 Missing and 2 partials ⚠️
... and 9 more

Additional details and impacted files

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##             alexreinking/rfactor-hoisting    #9416   +/-   ##
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  Lines                                    ?    79998           
  Branches                                 ?    19531           
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  Hits                                     ?    55755           
  Misses                                   ?    18295           
  Partials                                 ?     5948           

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