Migrate 9p file system to new backend - #1113

Similar to other migrations, it is easiest to review one commit at a time.

GPT-5.6 Sol: Requested changes—found 5 issues:

• High:  O_CREAT | O_EXCL | O_TRUNC  may truncate an existing file before returning  AlreadyExists  ( nine_p/mod.rs:417-425 ,  resolver.rs:526-530 ).
• High: Write-opening a directory can panic via  unimplemented!()  ( nine_p/mod.rs:355-358 ).
• Medium: Concurrent appenders can overwrite each other ( nine_p/mod.rs:500-513 ,  resolver.rs:661-671 ).
• Medium: Seeking beyond EOF is rejected, breaking sparse-file creation ( nine_p/mod.rs:486-488 ,  resolver.rs:716-720 ).
• Medium: A malformed 9P walk response can trigger an assertion panic ( nine_p/mod.rs:324-332 ).

Thanks Weidong, quick notes on the issues your agent raised:

1. same as the one on a prior PR, I already have a TODO on this
2. unimplemented!() panics are expected as "fixed in future PR"
3. this is already tracked at the resolver
4. this is already tracked in the resolver
5. yes there is an assertion there, that is literally stating "the server is working as intended", surviving a malformed 9P server is not a thing we need to protect against, right? Either way would be outside the purview of this PR.

Note to myself: need to rebase after #1111 is merged before I merge

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

--- 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::nine_p::FileSystem, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/29132b2d2bd82ddaf2eafc3df584239b8388c626/litebox/src/fs/nine_p/mod.rs:271