fix(cmake): adopt DFLASH27B_* options left from before the rename

[Graffioh]

The luce rename (#748) moved every CMake option from DFLASH27B_* (and DFLASH_GPU_SAMPLER) to LUCE_* with no migration. A build directory configured under the old names keeps them in its cache, so its next configure silently falls back to the defaults:

• An R9700 build dir with DFLASH27B_HIP_ARCHITECTURES=gfx1201 got LUCE_HIP_ARCHITECTURES empty and compiled ggml for the gfx1151 default. Unit tests still passed because they launch no kernels, but a live luce_server segfaulted in its first HIP kernel launch (rms_norm_mul_f32_cuda in the PFlash drafter forward) and logged nothing.
• A DFLASH27B_GPU_BACKEND=hip dir would switch to CUDA.

CI configures fresh dirs with the new names, so it never sees this.

Fix: each renamed user option adopts its old cached value once, when the new option is unset, empty, or still at its declared default (the list in the shim mirrors each option's declaration). It prints a rename warning and then drops the old entry. Checking against the default matters because a single configure caches LUCE_GPU_BACKEND=cuda and the ON switches. So an old -D on an existing dir, or a stale old entry in a dir reconfigured once after the rename, is adopted too. A LUCE_* value set away from its default wins, and the old flag is reported as ignored. The adopted entry keeps the new option's cache type, or BOOL for an untyped ON/OFF option that this backend doesn't declare (LUCE_GPU_SAMPLER on HIP).

Checked with two real configures (HIP, ROCm 7.2.4):

• A fresh dir with -DDFLASH27B_GPU_BACKEND=hip -DDFLASH27B_HIP_ARCHITECTURES=gfx1201 gives --offload-arch=gfx1201 for ggml-hip and luce_common.
• A dir first configured without an arch (gfx1151 default), then reconfigured with -DDFLASH27B_HIP_ARCHITECTURES=gfx1201, switches to --offload-arch=gfx1201.
• A dir with defaults cached (LUCE_TESTS:BOOL=ON), reconfigured with -DDFLASH27B_TESTS=OFF -DDFLASH27B_HIP_ARCHITECTURES=gfx1201, gives LUCE_TESTS:BOOL=OFF and 227 gfx1201 / 0 gfx1151 offload-arch flags.
• A fresh HIP dir with -DDFLASH_GPU_SAMPLER=OFF gives LUCE_GPU_SAMPLER:BOOL=OFF, and a second plain reconfigure prints no rename warnings.
• A standalone copy of the shim covers GPU_BACKEND: a cached cuda default plus -DDFLASH27B_GPU_BACKEND=hip, or a stale old hip entry, both give hip. A user-set LUCE_HIP_ARCHITECTURES=gfx1100 beats an old gfx1201. (This box has no nvcc, so a real CUDA-default configure can't finish here.)

🤖 Generated with Claude Code

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[davide221]

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