feat: implement & test open_at

[Sl1mb0]

Closes #336

I followed the specification described here. Given that we will eventually need to test this against the wasi test-suite, any differences between the aforementioned spec and the test-suite will favor the test-suite; so significant changes may need to be made in the future.

Describe your proposed changes here.

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[Copilot]

Pull request overview

Implements POSIX/WASI-like open_at behavior in the in-memory VFS so guests can open existing nodes and create/truncate files (a prerequisite for enabling host-side FS writes), and updates unit/integration tests to reflect the new semantics.

Changes:

• Refactors VFS path resolution and expands VfsCtxView::open_at to handle CREATE, EXCLUSIVE, DIRECTORY, and TRUNCATE.
• Adds extensive unit tests around open_at behavior and updates integration test expectations/snapshots.
• Makes Limiter::grow take &self (internally synchronized), simplifying ownership/mutability in component setup.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
host/src/vfs/mod.rs	Implements new open_at semantics, factors path traversal helper, and adds unit tests.
host/src/limiter.rs	Changes grow to &self to allow shared use behind a mutex.
host/src/component.rs	Adjusts limiter initialization to match the new Limiter::grow signature/usage.
host/tests/integration_tests/python/runtime/fs.rs	Updates Python FS integration test to exercise create-on-open behavior.
host/tests/integration_tests/evil/fs.rs	Updates large snapshot expectations for new VFS/open behavior.

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[Sl1mb0]

This is a massive PR but most of the diffs are in the tests; the actual implementation is fairly minimal.