Remove filesystem migration compatibility layers

[Weidong Cui (wdcui)]

This PR removes LiteBox’s legacy filesystem resolver and compatibility exports, temporary migration tests and test-only dependencies, obsolete Linux shim transport code, and transitional broker-core test scaffolding, leaving broker-owned filesystem APIs and tests as the sole implementation path.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- 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-ulitebox/3851b376ec8c12ee7fd51a8c65787781bbc534f2/litebox/src/fs/errors.rs:220
  enum litebox::fs::SeekWhence, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3851b376ec8c12ee7fd51a8c65787781bbc534f2/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-ulitebox/3851b376ec8c12ee7fd51a8c65787781bbc534f2/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::resolver::ResolvedPath, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3851b376ec8c12ee7fd51a8c65787781bbc534f2/litebox/src/fs/resolver.rs:152
  struct litebox::fs::resolver::Context, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3851b376ec8c12ee7fd51a8c65787781bbc534f2/litebox/src/fs/resolver.rs:74
  struct litebox::fs::DirEntry, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3851b376ec8c12ee7fd51a8c65787781bbc534f2/litebox/src/fs/mod.rs:303
  struct litebox::fs::NodeInfo, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3851b376ec8c12ee7fd51a8c65787781bbc534f2/litebox/src/fs/mod.rs:291
  struct litebox::fs::resolver::Resolver, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3851b376ec8c12ee7fd51a8c65787781bbc534f2/litebox/src/fs/resolver.rs:26
  struct litebox::fs::OFlags, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3851b376ec8c12ee7fd51a8c65787781bbc534f2/litebox/src/fs/mod.rs:161
  struct litebox::fs::FileStatus, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3851b376ec8c12ee7fd51a8c65787781bbc534f2/litebox/src/fs/mod.rs:265
  struct litebox::fs::Mode, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3851b376ec8c12ee7fd51a8c65787781bbc534f2/litebox/src/fs/mod.rs:114
  struct litebox::fs::UserInfo, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3851b376ec8c12ee7fd51a8c65787781bbc534f2/litebox/src/fs/mod.rs:282