fix(shim): error when secondary executable is missing

[CalvinAllen]

Summary

• When a secondary-executable shim (e.g. uv, pip, npm) cannot be located in the active runtime version's install tree, the shim previously fell back to the primary runtime binary — silently turning uv --version into python --version.
• Extract the lookup into a shared shim.FindSecondaryExecutable helper that returns an explicit error, and have both the shim entrypoint and dtvem which surface a clear "not available in " message instead of falling back.
• This also de-duplicates the previously-identical adjustExecutablePath functions in src/cmd/shim/main.go and src/cmd/which.go.

Resolves #254

Behavior change (verified locally)

Before:

$ DTVEM_VERBOSE=1 ~/.dtvem/shims/uv.exe --version
Shim invoked: uv
Mapped to runtime: python
Resolved version: 3.8.9
Base executable path: ...\python\3.8.9\python.exe
Final executable path: ...\python\3.8.9\python.exe
Python 3.8.9

After:

$ DTVEM_VERBOSE=1 ~/.dtvem/shims/uv.exe --version
Shim invoked: uv
Mapped to runtime: python
Resolved version: 3.8.9
Base executable path: ...\python\3.8.9\python.exe
Secondary executable lookup failed: secondary executable not found: uv
✗ 'uv' is not available in Python 3.8.9
→ This shim exists because another installed Python version provides it.
→ Install 'uv' for the active version, or switch to a version that has it.
dtvem shim error: uv not available in Python 3.8.9

Test plan

• ./rnr check (format, lint, full test suite) passes
• New unit tests in src/internal/shim/executable_test.go cover: found alongside runtime, found in Scripts/, found in ../Scripts/, .cmd preferred over .exe, not-found returns ErrSecondaryExecutableNotFound
• End-to-end: rebuilt shim, renamed to uv.exe, ran against a Python version that does not have uv — confirmed clear error
• End-to-end: rebuilt shim, renamed to python.exe — confirmed primary-runtime path still works
• CI passes on Windows / macOS / Linux

Note on related work

This does not fix #253 (shim-map cross-version contamination) — that's a separate change. However, fixing this silent-fallback bug significantly mitigates the user-facing impact of #253: users now see a clear error rather than the runtime binary running in place of the missing command.