Phase C+11.1 — `cache:/access` / `cache:/recent` dir-vs-file fix
================================================================

File: crates/xenia-kernel/src/exports.rs
Function: open_cache_file

Hunk 1: replace the post-existing-file-wins block with a
disposition-gated mkdir + STATUS_OBJECT_NAME_NOT_FOUND on miss.

Pre-fix (Phase C+11 HEAD):

    let host_exists_as_dir = host_path.is_dir();
    let host_exists_as_file = host_path.is_file();
    let is_dir_open = host_exists_as_dir
        || (!host_exists_as_file && !want_non_dir && want_dir);
    if is_dir_open {
        // For non-existent paths the guest wants us to create as a
        // directory, mkdir-p; canary's HostPathDevice does the same
        // when FILE_DIRECTORY_FILE is set on a kCreate disposition.
        if want_dir && !host_path.exists() {
            if let Err(e) = std::fs::create_dir_all(host_path) {
                ...STATUS_UNSUCCESSFUL...
            }
        }
        // ... falls through to SUCCESS / handle alloc

Post-fix (Phase C+11.1):

    let host_exists_as_dir = host_path.is_dir();
    let host_exists_as_file = host_path.is_file();
    let is_dir_open = host_exists_as_dir
        || (!host_exists_as_file && !want_non_dir && want_dir);
    if is_dir_open {
        // Phase C+11.1 — only create the host directory when the
        // disposition is *create-capable*. Mirrors canary's
        // `VirtualFileSystem::OpenFile` (virtual_file_system.cc:265-273):
        // for `FileDisposition::kOpen`/`kOverwrite` on a non-existent
        // path the function returns `X_STATUS_OBJECT_NAME_NOT_FOUND`
        // *before* any `CreatePath` call — i.e. mkdir is never invoked
        // on these dispositions. The pre-fix code (Phase C+11) called
        // `create_dir_all` whenever `want_dir && !host_path.exists()`,
        // so Sylpheed's cold-boot probes for `cache:/access`,
        // `cache:/ignore`, `cache:/recent` (disp=1, opts=0x7) succeeded
        // and produced spurious host directories. Canary instead
        // returns NOT_FOUND, after which Sylpheed re-creates these as
        // FILES via `disp=5` + `FILE_NON_DIRECTORY_FILE`.
        //
        // Create-capable dispositions (mkdir OK):
        //   0 FILE_SUPERSEDE
        //   2 FILE_CREATE
        //   3 FILE_OPEN_IF
        //   5 FILE_OVERWRITE_IF
        // Non-create dispositions (must miss when path is absent):
        //   1 FILE_OPEN
        //   4 FILE_OVERWRITE
        let disp_is_create_capable = matches!(
            create_disposition,
            FILE_SUPERSEDE | FILE_CREATE | FILE_OPEN_IF | FILE_OVERWRITE_IF
        );
        if !host_path.exists() {
            if !disp_is_create_capable {
                if handle_out != 0 {
                    mem.write_u32(handle_out, 0);
                }
                write_io_status_block(
                    mem,
                    io_status_block,
                    STATUS_OBJECT_NAME_NOT_FOUND as u32,
                    0,
                );
                tracing::info!(
                    "cache open (dir) MISS path={:?} disp={} opts={:#x} -> NOT_FOUND",
                    guest_path,
                    create_disposition,
                    create_options
                );
                return STATUS_OBJECT_NAME_NOT_FOUND;
            }
            // create-capable + want_dir → mkdir-p the directory.
            if want_dir {
                if let Err(e) = std::fs::create_dir_all(host_path) {
                    ...STATUS_UNSUCCESSFUL...
                }
            }
        }
        // ... falls through to SUCCESS / handle alloc as before

Hunk 2 (tests): two new unit tests added in
crates/xenia-kernel/src/exports.rs after
`cache_top_level_manifests_create_as_files`:

  - cache_open_directory_on_missing_path_returns_not_found
    Loops over the three cold-probe paths Sylpheed actually emits
    (cache:\\access, cache:\\ignore, cache:\\recent) and asserts
    NtCreateFile + disp=1 + opts=0x7 returns NOT_FOUND, writes
    handle=0, and leaves no host entry on disk.

  - cache_disp5_after_disp1_miss_creates_file
    Pins the canary two-call sequence: cold disp=1 returns NOT_FOUND;
    immediately following disp=5 + opts=0x60 (FILE_NON_DIRECTORY_FILE)
    succeeds and produces a host FILE.

LOC summary: ~30 added in open_cache_file (mkdir gate + NOT_FOUND
return branch + comments), ~6 removed (the unconditional mkdir
flow); ~88 lines of test code for the two new tests.