fix: resolve the input path against the root before making it virtual

[msallin]

Problem

A Typst virtual path only accepts forward slashes. SystemWorld::new handed the caller's input path to VirtualPath::new verbatim and unwrapped the result, so on Windows an ordinary relative path such as templates\letter.typ failed to convert and panicked.

The panic happens inside create_compiler, which is extern "C" and has no unwind guard. A panic cannot unwind out of an extern "C" frame, so the process aborts rather than the call failing. Running the new tests against develop:

13: create_compiler
14: input_path::compiler_for_file
thread caused non-unwinding panic. aborting.
process didn't exit successfully (signal: 6, SIGABRT: process abort signal)

For a service on Windows, or any layout that sets an explicit root and keeps templates in a subfolder, this terminates the host process on the first compile with no managed exception. A .. component in the path does the same.

Change

Relative paths are joined onto the project root, then converted with VirtualPath::virtualize, which splits on the platform separator and performs the root containment check itself. A path that escapes the root now returns an error through the existing channel instead of panicking.

The manual to_str UTF-8 check is dropped because virtualize reports that case as VirtualizeError::Utf8. The <main> unwrap becomes an expect naming the invariant.

This does not add an unwind guard to create_compiler; that is a separate concern and other panic sources remain.

Tests

New src/typst_core/tests/input_path.rs covers a nested relative path, the equivalent absolute path, a relative path escaping the root, and an absolute path outside the root. New tests in Tests.cs cover the same cases through the managed API.

All four Rust tests abort the test binary against develop and pass with the change.

cargo test --release    7 passed
dotnet test            48 passed

[msallin]

Superseded by evolvedlight#37; the fix belongs upstream.