Migrate Linux file clients to the broker

[Weidong Cui (wdcui)]

This PR routes Linux shim file, mmap, pipe, loader, and process operations through broker-backed LiteBox APIs, requires hosted Linux runners to use the broker-owned filesystem with pre-rewritten guests, updates runner tests and benchmark tooling, and explicitly disables SNP filesystem startup until a kernel broker platform exists.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure struct_pub_field_missing: pub struct's pub field removed or renamed ---

Description:
A publicly-visible struct has at least one public field that 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/v0.50.0/src/lints/struct_pub_field_missing.ron

Failed in:
  field insert_files of struct CliArgs, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4d0051ddf624b97c8619b59419e04cb5f3c0f77b/litebox_runner_linux_userland/src/lib.rs:55
  field initial_files of struct CliArgs, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4d0051ddf624b97c8619b59419e04cb5f3c0f77b/litebox_runner_linux_userland/src/lib.rs:59
  field rewrite_syscalls of struct CliArgs, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4d0051ddf624b97c8619b59419e04cb5f3c0f77b/litebox_runner_linux_userland/src/lib.rs:69
  field program_from_tar of struct CliArgs, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4d0051ddf624b97c8619b59419e04cb5f3c0f77b/litebox_runner_linux_userland/src/lib.rs:81

--- 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/v0.50.0/src/lints/inherent_method_missing.ron

Failed in:
  LinuxShimBuilder::default_fs, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4d0051ddf624b97c8619b59419e04cb5f3c0f77b/litebox_shim_linux/src/lib.rs:236
  LinuxShim::tcp_connection, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4d0051ddf624b97c8619b59419e04cb5f3c0f77b/litebox_shim_linux/src/lib.rs:345

--- failure method_parameter_count_changed: pub method parameter count changed ---

Description:
A publicly-visible method now takes a different number of parameters, not counting the receiver (self) parameter.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#fn-change-arity
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/method_parameter_count_changed.ron

Failed in:
  litebox_shim_linux::LinuxShim::load_program takes 5 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4d0051ddf624b97c8619b59419e04cb5f3c0f77b/litebox_shim_linux/src/lib.rs:273, but now takes 4 parameters in /home/runner/work/litebox/litebox/litebox_shim_linux/src/lib.rs:253

--- failure module_missing: pub module removed or renamed ---

Description:
A publicly-visible module cannot be imported by its prior path. A `pub use` may have been removed, or the module may have been renamed, removed, or made non-public.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/module_missing.ron

Failed in:
  mod litebox_shim_linux::transport, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4d0051ddf624b97c8619b59419e04cb5f3c0f77b/litebox_shim_linux/src/transport.rs:4

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct 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/v0.50.0/src/lints/struct_missing.ron

Failed in:
  struct litebox_shim_linux::transport::ShimTransport, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/4d0051ddf624b97c8619b59419e04cb5f3c0f77b/litebox_shim_linux/src/transport.rs:55