Store user pointers as address-only types instead of platform pointers

[jstarks]

This PR changes how the Linux shim represents user pointers, so that Platform is no longer viral across types that merely hold a pointer value.

Previously, litebox_shim_linux and litebox_common_linux stored user pointers using the platform's associated pointer types (Platform::RawConstPointer/RawMutPointer). Because those types are parameterized by the platform, every struct, enum, and signature that carried a user pointer had to carry a Platform (or RawPointerProvider) parameter -- even when it never dereferenced anything. This is the main obstacle to eventually dropping litebox_platform_multiplex from the shim in favor of explicit Platform generic bounds.

This PR introduces address-only UserPtr<T>/UserPtrMut<T> (transparent usize wrappers) in litebox_common_linux and uses them everywhere a user pointer is stored. The platform only re-enters at the handful of sites that actually access memory: the access methods take the platform as an explicit witness (e.g. ptr.read_at_offset::<Platform>(..)), converting back to the real platform pointer via to_platform_ptr. As a result, Platform shows up only where memory is genuinely read or written, which is both clearer and the setup needed to make the shim generic over the platform.

Behavior is unchanged; this is purely a representation refactor.

[jaybosamiya-ms]

Thanks John! Overall looks good to me, just a few minor nits to fix up before merging

[jaybosamiya-ms]

Also, since semver-checks CI bot does not fire on branches from forks, here's its output as of bac25fe (basically expected tweaks):

click to unfold

--- failure auto_trait_impl_removed: auto trait no longer implemented ---

Description:
A public type has stopped implementing one or more auto traits. This can break downstream code that depends on the traits being implemented.
        ref: https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/auto_trait_impl_removed.ron

Failed in:
  type UserMsgHdr is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1847
  type UserMsgHdr is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1847
  type IoReadVec is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:359
  type IoReadVec is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:359
  type IoReadVec is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:359
  type IoReadVec is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:359
  type UserMmsgHdr is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1876
  type UserMmsgHdr is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1876
  type IoWriteVec is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:351
  type IoWriteVec is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:351
  type SigSetPack is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1840
  type SigSetPack is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1840
  type IoctlArg is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:718
  type IoctlArg is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:718
  type ArchPrctlArg is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1069
  type ArchPrctlArg is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1069
  type FcntlArg is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:581
  type FcntlArg is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:581
  type TimeParam is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2997
  type TimeParam is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2997
  type FutexArgs is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1683
  type FutexArgs is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1683
  type PrctlArg is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1763
  type PrctlArg is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1763
  type SyscallRequest is no longer Send, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1938
  type SyscallRequest is no longer Sync, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1938

--- failure method_requires_different_generic_type_params: method now requires a different number of generic type parameters ---

Description:
A method now requires a different number of generic type parameters than it used to. Uses of this method that supplied the previous number of generic types will be broken.
        ref: https://doc.rust-lang.org/reference/items/generics.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/method_requires_different_generic_type_params.ron

Failed in:
  litebox_common_linux::TimeParam::read takes 1 generic types instead of 0, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:3028
  litebox_common_linux::TimeParam::write takes 1 generic types instead of 0, in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:3057

--- failure type_allows_fewer_generic_type_params: type now allows fewer generic type parameters ---

Description:
A type now allows fewer generic type parameters than it used to. Uses of this type that supplied all previously-supported generic types will be broken.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-parameter-no-default
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/type_allows_fewer_generic_type_params.ron

Failed in:
  Struct UserMsgHdr allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1847
  Struct IoReadVec allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:359
  Struct IoReadVec allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:359
  Struct UserMmsgHdr allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1876
  Struct IoWriteVec allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:351
  Struct SigSetPack allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1840
  Enum IoctlArg allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:718
  Enum ArchPrctlArg allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1069
  Enum FcntlArg allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:581
  Enum TimeParam allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2997
  Enum FutexArgs allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1683
  Enum PrctlArg allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1763
  Enum SyscallRequest allows 1 -> 0 generic types in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1938

[jaybosamiya-ms]

Pushed fixes for the minor nits; lgtm