build(ml-alpha): multi-cubin build.rs + placeholder kernels + local MappedF32Buffer

Cargo.toml: drops gbdt; adds memmap2 + approx; keeps ml-core only
(cannot depend on ml: would cycle since ml depends on ml-alpha for
the Mamba2 gate baseline).

build.rs: compiles 7 cubins (mamba2_alpha + 6 new placeholders)
with -O3 --use_fast_math --ftz --fmad. Skips kernels whose source
isn't present yet so partial check-ins work. Every env::var paired
with rerun-if-env-changed per the canonical build pearl.

src/pinned_mem.rs: local copy of MappedF32Buffer (mirrors
ml::cuda_pipeline::mapped_pinned::MappedF32Buffer). Drives the only
permitted CPU<->GPU path per feedback_no_htod_htoh_only_mapped_pinned.
Eventually the move-to-ml-core refactor will deduplicate; out of
scope for the Phase A branch.

Addendum: updates the import path to ml_alpha::pinned_mem.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/superpowers/plans/2026-05-16-ml-alpha-phase-a-api-addendum.md

@@ -22,7 +22,7 @@ use cudarc::driver::{
     CudaContext, CudaStream, CudaModule, CudaFunction, CudaSlice,
     DevicePtr, DevicePtrMut, LaunchConfig, PushKernelArg, DriverError,
 };
-use ml::cuda_pipeline::mapped_pinned::MappedF32Buffer;
+use crate::pinned_mem::MappedF32Buffer;  // local copy — see pinned_mem.rs header
 use ml_core::device::MlDevice;
 ```