Fix ir run on rig-managed Windows

[cderv]

On a rig-managed Windows machine, ir run fails before reaching R, and even with an explicit --r-version the .qmd render aborts in quarto. Two separate Windows-only bugs (#32).

Default-R resolution hits the .bat shim

With no --r-version, resolution falls back to the bare name Rscript. On a rig-managed Windows box the only Rscript on PATH is Rscript.bat (from rig system make-links), and std::process::Command resolves .exe only — it won't spawn a .bat:

could not find `Rscript` on PATH. Install R, or set IR_RSCRIPT to its path.

The default install's real Rscript.exe is now resolved from rig list --json ("default": true → its binary), the same mechanism the --r-version path already uses. IR_RSCRIPT stays the top override; non-rig systems and Unix fall back to bare Rscript unchanged.

Canonicalised script paths break quarto

The script path was canonicalised for CWD-independence. On Windows fs::canonicalize returns a \\?\C:\... verbatim path that Rscript tolerates but quarto's Deno expandGlobSync cannot stat:

ERROR: (os error 123): stat '\\?\C:\Users\...\e2e.qmd'

The path is now passed through untouched. R and quarto both inherit ir's working directory, so a relative path resolves exactly as the user typed it — no canonicalisation, no verbatim prefix. An existence check keeps the clear "cannot read script" error.

Verified locally on a rig-managed Windows box: a default-R .R run and a --r-version .qmd render both succeed with the fixes.

Fixes #32

[cderv]

The test (windows-latest) failure here is pre-existing on main since #39 — the same 5 fixture tests fail identically there (e.g. runs 26959944760, 26958971343), independent of this change. Root-caused in #46: sessioninfo system2(env=) crashes on Windows (r-lib/sessioninfo#122), a quarto cross-drive --output-dir join, and reticulate Python discovery on the runner.

None of those touch this PR's diff (main.rs/rig.rs). And the CI job uses r-lib setup-r (real Rscript.exe), so it never exercises the rig .bat shim or default-R resolution these fixes target — that path was validated locally on a rig-managed Windows box.