Remove filesystem migration compatibility layers

[Weidong Cui (wdcui)]

This PR removes the legacy LiteBox resolver and filesystem engine surface, temporary migration tests and adapters, obsolete Linux transport code, and transitional broker-core scaffolding so the cumulative stacked branch is source-identical to #1334.

[github-actions]

ℹ️ Note: This semver check was run against the wdcui/ulitebox/broker-files-10-windows-clients branch, not main or ulitebox.

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure enum_missing: pub enum removed or renamed ---

Description:
A publicly-visible enum cannot be imported by its prior path. A `pub use` may have been removed, or the enum 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/enum_missing.ron

Failed in:
  enum litebox::fs::errors::WalkError, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_files_10_windows_clients/5ae9ce6e88a96b1b4012a72ad5ed83251454a661/litebox/src/fs/errors.rs:216
  enum litebox::fs::SeekWhence, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_files_10_windows_clients/5ae9ce6e88a96b1b4012a72ad5ed83251454a661/litebox/src/fs/mod.rs:249

--- 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::fs::resolver, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_files_10_windows_clients/5ae9ce6e88a96b1b4012a72ad5ed83251454a661/litebox/src/fs/resolver.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::fs::Mode, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_files_10_windows_clients/5ae9ce6e88a96b1b4012a72ad5ed83251454a661/litebox/src/fs/mod.rs:114
  struct litebox::fs::OFlags, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_files_10_windows_clients/5ae9ce6e88a96b1b4012a72ad5ed83251454a661/litebox/src/fs/mod.rs:161
  struct litebox::fs::FileStatus, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_files_10_windows_clients/5ae9ce6e88a96b1b4012a72ad5ed83251454a661/litebox/src/fs/mod.rs:265
  struct litebox::fs::resolver::Resolver, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_files_10_windows_clients/5ae9ce6e88a96b1b4012a72ad5ed83251454a661/litebox/src/fs/resolver.rs:26
  struct litebox::fs::UserInfo, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_files_10_windows_clients/5ae9ce6e88a96b1b4012a72ad5ed83251454a661/litebox/src/fs/mod.rs:282
  struct litebox::fs::resolver::ResolvedPath, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_files_10_windows_clients/5ae9ce6e88a96b1b4012a72ad5ed83251454a661/litebox/src/fs/resolver.rs:152
  struct litebox::fs::resolver::Context, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_files_10_windows_clients/5ae9ce6e88a96b1b4012a72ad5ed83251454a661/litebox/src/fs/resolver.rs:74
  struct litebox::fs::DirEntry, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_files_10_windows_clients/5ae9ce6e88a96b1b4012a72ad5ed83251454a661/litebox/src/fs/mod.rs:303
  struct litebox::fs::NodeInfo, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_files_10_windows_clients/5ae9ce6e88a96b1b4012a72ad5ed83251454a661/litebox/src/fs/mod.rs:291