[Weiteng Chen (CvvT)]

This PR adds storage that lets a file's contents live in host shared memory, so that later PRs can map broker files into runners with MAP_SHARED. In-mem fs supports this by moving a file's contents into a SharedFileMemory the first time it's called and returns the host object as a SharedFile. From then on, reads, writes and truncations all use that storage, and writes behave like pwrite.

The userland broker provides the storage on each host. Linux uses a memfd while Windows uses a delete-on-close temporary file. Windows can't shrink a file while it's mapped, so the broker tracks the file's size itself, zeroes the bytes a refused shrink cuts off, and shrinks the host file once nothing maps it.

One limitation is that we need one host descriptor or handle per shared file and the total number of host fd is limited. Additionally, because we don't know when the shared file is no longer needed, we never close it.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure enum_no_repr_variant_discriminant_changed: enum variant had its discriminant change value ---

Description:
The enum's variant had its discriminant value change. This breaks downstream code that used its value via a numeric cast like `as isize`.
        ref: https://doc.rust-lang.org/reference/items/enumerations.html#assigning-discriminant-values
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/enum_no_repr_variant_discriminant_changed.ron

Failed in:
  variant TruncateError::Io 5 -> 7 in /home/runner/work/litebox/litebox/litebox/src/fs/errors.rs:120

--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/enum_variant_added.ron

Failed in:
  variant TruncateError:Busy in /home/runner/work/litebox/litebox/litebox/src/fs/errors.rs:116
  variant TruncateError:NoSpace in /home/runner/work/litebox/litebox/litebox/src/fs/errors.rs:118

--- failure enum_no_repr_variant_discriminant_changed: enum variant had its discriminant change value ---

Description:
The enum's variant had its discriminant value change. This breaks downstream code that used its value via a numeric cast like `as isize`.
        ref: https://doc.rust-lang.org/reference/items/enumerations.html#assigning-discriminant-values
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/enum_no_repr_variant_discriminant_changed.ron

Failed in:
  variant TruncateError::Io 5 -> 7 in /home/runner/work/litebox/litebox/litebox_broker_core/src/fs/errors.rs:125

--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/enum_variant_added.ron

Failed in:
  variant TruncateError:Busy in /home/runner/work/litebox/litebox/litebox_broker_core/src/fs/errors.rs:121
  variant TruncateError:NoSpace in /home/runner/work/litebox/litebox/litebox_broker_core/src/fs/errors.rs:123
  variant WriteError:NoSpace in /home/runner/work/litebox/litebox/litebox_broker_core/src/fs/errors.rs:77