feat: implement & test write

[Sl1mb0]

• feat: implement write (3rd time's the charm)

[Copilot]

Pull request overview

Implements write support for the in-memory VFS (VfsCtxView), replacing the previous ReadOnly trap and adding a suite of unit tests validating POSIX-like behaviors around writing.

Changes:

• Implement HostDescriptor::write for regular files, including file extension, zero-fill gaps, and memory-limit enforcement via the limiter.
• Add comprehensive unit tests for write covering success cases, offsets, EOF extension, directory errors, permission errors, and insufficient-memory behavior.

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[Copilot]

offset is a Filesize (WASI type) but is cast with offset as usize. On 32-bit hosts or when guests pass offsets > usize::MAX, this truncates and can write to the wrong location rather than returning an error. Please use a fallible conversion (usize::try_from(offset)) and trap with an appropriate error code when it doesn’t fit.

Suggested change

	let offset = offset as usize;
	let offset = usize::try_from(offset)
	.map_err(|_| FsError::trap(ErrorCode::Overflow))?;

[Copilot]

write_end = offset.saturating_add(buffer_len) hides arithmetic overflow. If offset + buffer_len overflows usize, this will silently clamp to usize::MAX, potentially attempting an enormous Vec::resize or producing incorrect accounting. Prefer checked_add and return an error when the requested end offset can’t be represented.

Suggested change

	let write_end = offset.saturating_add(buffer_len);
	let write_end = offset
	.checked_add(buffer_len)
	.ok_or_else(|| FsError::trap(ErrorCode::InsufficientMemory))?;

[Copilot]

The file extension logic does two resizes (resize(offset, 0) and later resize(write_end, 0)). Since write_end is already known, this can be simplified to a single resize to write_end (which also zero-fills any gap) to reduce reallocations and keep the control flow simpler.

Suggested change

	// Extend the file with zeros if writing past the current end
	// Per POSIX: Writing past EOF extends the file
	if offset > current_len {
	content.resize(offset, 0);
	}
	// Ensure we have enough capacity for the write
	// Extend the file with zeros if writing past the current end.
	// A single resize to write_end is sufficient: Vec::resize zero-fills
	// any newly allocated region, including any gap between the previous
	// end-of-file and the write offset, matching POSIX semantics.