#1554 verify: remove the unused resident_plan_par_kernel (CUDA and SYCL); INTEL.md no longer names STRATA_PLAN_PARALLEL

Beschreibung

## Summary
Two leftovers of the parallel resident plan. No behaviour change.

- `resident_plan_par_kernel` (the one-block parallel plan from S26) is defined in `src/kernels/cuda/verify_kernels.cu` and in the SYCL port's `sycl/src/kernels/cuda/verify_kernels.dp.cpp`, and launched in neither: `resident_plan_kernel` has grouped the entries in parallel since #783 PR-e (the prefix scan `resident_plan_parity` checks), and the port takes that kernel since its re-migration to 0.1.40.1. nvcc reports it: `warning #177-D: function "resident_plan_par_kernel" was declared but never referenced`.
- `docs/INTEL.md` still lists `STRATA_PLAN_PARALLEL=0` among the switches to the old paths. No code reads it any more (it went with the port's own plan in the re-migration), so a user who sets it gets nothing.

## What changed
- `src/kernels/cuda/verify_kernels.cu`: `resident_plan_par_kernel` and its comment removed (65 lines).
- `sycl/src/kernels/cuda/verify_kernels.dp.cpp`: the same function removed (90 lines).
- `docs/INTEL.md`: `STRATA_PLAN_PARALLEL=0` taken out of the switch list, with one sentence on why the plan has no switch. `STRATA_GR_DOWN_DIRECT` and `STRATA_MMVQ_WIDE_K`, the other two on that line, are still read by the port.

## Checked
On 0.1.40.4, Windows (MSVC, CUDA 13): `strata` builds, without the #177-D warning; `resident_plan_parity`: 641 cases, 0 failures. `git grep resident_plan_par_kernel` and `git grep STRATA_PLAN_PARALLEL` find nothing afterwards. Not built with the SYCL toolchain (no oneAPI here); the removed function had no caller there either.

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