PERF: Quantify `-mprefer-vector-width=256` as a required companion to `-march=x86-64-v4`

`CMake/ITKSetStandardCompilerFlags.cmake:163-172` advises users on AVX-512
hardware to add `-mprefer-vector-width=256`, but that advice is unquantified and
the two issues it cites (#2634, #1939) are both closed. Local benchmarks suggest
the advice is correct and should be promoted to documented, measured guidance.

Measured evidence — Sapphire Rapids Resample sweep

Host: Intel Xeon w7-3545 (Sapphire Rapids), GCC 13.3. Four-config Resample
benchmark sweep:

| Variant | `-march=x86-64-v4` vs baseline |
|---|---|
| Affine-only | **+17% to +20%** |
| BSpline-dominated | **−12% to −17%** |

The regression is not localized to one hot function. The v4 build emits
**53,063 zmm instructions across 4,977 functions**, versus 162 zmm instructions
at baseline. That breadth keeps the CPU in the AVX-512 license-throttled P-state
essentially all the time (throttle persistence measured at ~670 µs after the
last wide instruction). BSpline `TransformPoint` is memory-latency-bound, so
wider vectors buy it nothing while the lower clock costs it directly.

Proposed work

1. Re-run the sweep with `-march=x86-64-v4 -mprefer-vector-width=256`.
Hypothesis: recovers the Affine speedup without the BSpline regression.
2. Repeat on at least one non-Intel AVX-512 part (Zen 4/5, which has no
license-based frequency throttle) to determine whether the guidance is
Intel-specific or universal.
3. Survey whether **any** ITK kernel family measurably benefits from 512-bit
width. If none does, promote the CMake comment to explicit documentation
that `-mprefer-vector-width=256` should always accompany `-march=x86-64-v4`,
and update `CMake/ITKSetStandardCompilerFlags.cmake` to say so with the
measurement attached.

Note that ITK sets no architecture flags by default; this is guidance for
downstream builders and packagers, not a default-behavior change.

Why the referenced issues are dead

- #2634 — CLOSED 2026-04-13.
- #1939 — CLOSED 2020-10-20.
- #6039 (PR) — closed, superseded by merged #6049 ("COMP: Remove hardcoded
`-march=corei7`, `-mtune=generic`, and dead code"). The `ITK_X86_64_ISA_LEVEL`
option that PR proposed does not exist on `main`.

So the CMake comment currently points readers at three dead references.

Start with CMake/ITKSetStandardCompilerFlags.cmake:163-172 and the cited closed issues, then reproduce the four-configuration Resample sweep with -march=x86-64-v4 -mprefer-vector-width=256. Repeat on a non-Intel AVX-512 system and survey ITK kernel families for width benefits; done means measured guidance is established and the CMake comment is updated with evidence and current references.

Written by the indexing model from the issue text.