feat(compat.spirv-headers): the headers a SPIR-V reader needs

[Sunrisepeak]

What

A new index entry, compat:spirv-headers@1.4.357.0 — the Khronos
spirv/unified1 tree — plus a workspace test member that decodes a hand-built
SPIR-V module header with it.

Why

A program that only submits SPIR-V to a driver needs nothing from this
package: the module is opaque data and vulkan_core.h describes the call that
takes it. A program that reads one needs the opcode and enum definitions.

llama.cpp's Vulkan backend is such a program — ggml-vulkan.cpp inspects the
modules its own shader generator produced, through
<spirv/unified1/spirv.hpp>. The index carried compat.vulkan-headers and not
this, so that consumer had nowhere in the ecosystem to get it and would have
resolved it from the host's /usr/include.

Split rather than folded into compat.vulkan-headers, following Khronos: they
are separate repositories on separate cadences, and a renderer that never
decodes SPIR-V should not acquire the decoder's headers.

Notes

• The licence is written as a set. The bulk is MIT; upstream's own LICENSE
  calls out files under CC-BY-4.0 and ships LICENSES/ carrying both texts.
• The CN mirror is published and verified byte-identical to the upstream
  archive, so the entry takes the table form rather than the plain-string
  fallback.
• The test decodes rather than compiles. A header-only package that resolves
  and exposes the wrong include root compiles nothing, so "it compiled" would
  pass on a package that delivered an empty tree.

Measured

$ cd tests/examples/spirv-headers && mcpp test
 Downloading compat.spirv-headers v1.4.357.0
   Compiling compat.spirv-headers v1.4.357.0
     Running bin/decode
compat.spirv-headers: SPIR-V 1.6 definitions present
 test result ok. 1 passed; 0 failed