refactor(dispatch): Generate the dispatch surface from a single declaration

[ahuber21]

The set of distance kernels compiled ahead of time -- extents x ISA levels -- was
written out by hand everywhere it was needed: three extern template blocks,
two per-architecture translation units, and supported_dim_list. Adding an
extent meant editing six lists in agreement, and the preceding commit is what
happens when one of them disagrees.

cmake/dispatch-surface.cmake is now the only place the extent list and the ISA
levels are written down. Everything else is derived from it:

• include/svs/core/distance/dispatch_surface.h, which drives every
  extern template, every explicit instantiation, and supported_dim_list
• the object library each level's translation unit is compiled into, and the
  instruction budget it is compiled at

The generated header is committed as well as generated, so that consuming the
headers with a bare -I include and no CMake keeps working.

Configure now prints the surface in full rather than a count -- every extent,
and for each level its enumerator, its instruction budget and its translation
unit:

-- Dispatch surface: 9 extents x 2 ISA levels
--   extents: 64 96 100 128 160 200 512 768 svs::Dynamic
--   level:   AVX_AVAILABILITY::AVX2 -march=haswell avx2.cpp
--   level:   AVX_AVAILABILITY::AVX512 -march=cascadelake avx512.cpp
--   not in the surface: NONE
--            dispatched to, but compiled by no translation unit, so every
--            consumer instantiates those kernels itself, at its own -march

AVX_AVAILABILITY::NONE is a real hole and is now named as one: the entry points
fall back to it, no translation unit compiles it, so every consumer builds those
kernels itself at whatever -march it happens to use. A baseline translation
unit is its own change, not this one.

Seven ctest tests replace what was a paragraph of nm commands in a review
comment. All of them derive their expectations from the declaration, so they hold
for an overridden surface too:

• dispatch_surface_probe, dispatch_entry_probe -- consumers built at
  -march=x86-64. One names every kernel in the surface directly, the other
  reaches them through the entry points; a declared-but-uninstantiated kernel is
  an undefined symbol in the first, and a kernel the entry points cannot reach is
  one in the second.
• dispatch_surface_linkage -- the archive defines every kernel the probe names,
  nothing more, and the probe instantiated none of them itself: 864 kernels.
• dispatch_surface_declaration -- the same 864, but counted from the three
  hand-written sources (the extent list, the type-pair macros, the enumerator
  order) without consulting the generated header, so a generator bug has nowhere
  to hide: 2 levels x 9 extents x 3 distances x {AVX2: 16, AVX512: 16} pairs.
• dispatch_instructions_avx2, dispatch_instructions_avx512 -- one per level,
  disassembling that level's object file against the instruction budget its
  runtime predicate promises. The AVX2 object must contain ymm and no zmm,
  mask or VNNI encodings; a level that emits what its predicate does not
  guarantee is an illegal-instruction fault on a host the dispatcher considers
  supported, which no symbol-table check can see.
• dispatch_surface_execution -- the only check that observes a run rather than
  a symbol table: gdb breaks on every level's kernel for one extent and reports
  which one a call through the entry points actually enters. A specialization
  that disappears behind an #if still links and still counts.

No behaviour change: the same 864 kernels, at the same instruction budgets, with
the same mangled names. compute() is still compute(); nothing about the call
chain or the runtime dispatch moves.

Part 2 of 4 of the ISA dispatching v2 milestone.

[ahuber21]

@copilot new or modified files should have 2026 in their copyright header. Fix

[Copilot]

@copilot new or modified files should have 2026 in their copyright header. Fix

Updated the headers in 408513c.