Demand paging and fallible memory copy for LVBS

[sangho2]

Adds demand paging support for user-space addresses on the LVBS platform, handling user-space page faults from both user-mode and kernel-mode contexts (e.g., syscall handler copying not-yet mapped or invalid user pages using memcpy_fallible). This PR resolves #637.

Changes

• ISR stubs check CS RPL to route user- and kernel-mode exceptions to the shim with different CPU context savings; kernel-mode page faults at user-space addresses trigger demand paging or address fixup
• VTL0 memory copies replaced with memcpy_fallible + RAII Vtl0MappedGuard; copy_from_vtl0_phys uses zerocopy::FromBytes for memory safety
• Per-CPU separate stacks for double-fault (TSS.IST1) and use-mode exception (TSS.RSP0)
• Shim interface extended with kernel_mode field on ExceptionInfo and ResumeKernelPlatform/ExceptionFixup variants on ContinueOperation

[wdcui]

I left some comments below.

[wdcui]

LGTM. Thank you!

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure constructible_struct_adds_field: externally-constructible struct adds field ---

Description:
A pub struct constructible with a struct literal has a new pub field. Existing struct literals must be updated to include the new field.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field ExceptionInfo.kernel_mode in /home/runner/work/litebox/litebox/litebox/src/shim.rs:129

--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/enum_variant_added.ron

Failed in:
  variant ContinueOperation:ResumeKernelPlatform in /home/runner/work/litebox/litebox/litebox/src/shim.rs:108
  variant ContinueOperation:ExceptionFixup in /home/runner/work/litebox/litebox/litebox/src/shim.rs:113

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/inherent_method_missing.ron

Failed in:
  PerCpuVariablesAsm::set_interrupt_stack_ptr, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/069b405897dc01ef4702843c27669c77ce2e443e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:240
  PerCpuVariablesAsm::get_interrupt_stack_ptr, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/069b405897dc01ef4702843c27669c77ce2e443e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:243
  PerCpuVariablesAsm::interrupt_stack_ptr_offset, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/069b405897dc01ef4702843c27669c77ce2e443e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:274

--- failure pub_module_level_const_missing: pub module-level const is missing ---

Description:
A public const is missing or renamed
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/pub_module_level_const_missing.ron

Failed in:
  INTERRUPT_STACK_SIZE in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/069b405897dc01ef4702843c27669c77ce2e443e/litebox_platform_lvbs/src/host/per_cpu_variables.rs:24

--- failure function_missing: pub fn removed or renamed ---

Description:
A publicly-visible function cannot be imported by its prior path. A `pub use` may have been removed, or the function itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/function_missing.ron

Failed in:
  function litebox_shim_optee::recycle_session_id, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/069b405897dc01ef4702843c27669c77ce2e443e/litebox_shim_optee/src/session.rs:273