feat: provide a Cage constructor instead of hand-written full-field struct literals

[Yaxuan-w]

Before submitting

• I searched existing issues and discussions for similar proposals.

Problem or use case

Cage has 18 fields and no constructor. Every site that needs one writes out the full literal by hand. There are three today, and #1338 adds a fourth:

• rawposix init.rs
• rawposix fork syscall implementation
• cage lib internal test

Today's design will cause adding a field to Cage breaks unrelated crates' tests. With #1338 merged, a new field in src/cage/src/cage.rs fails to compile typemap's test suite.

Proposed solution

Add a constructor to the cage crate. For example:

impl Cage {
    /// Creates a cage with the given identity and cwd; every other field
    /// starts empty/zeroed.
    pub fn new(cageid: u64, parent: u64, cwd: PathBuf) -> Self { ... }
}

Alternatives considered

No response

Additional context

No response