fix(shim): dispatch Windows executables from version dir root, respect NVM_DIR

[mose-zm]

Summary

Fixes the tracked P0 (note/p0-windows-shim-bin-path.md): on Windows, node/npm/npx via PATH were broken in Full Shim mode — every dispatch through shims\*.cmd failed with nvm: node not found.

Root cause (empirically reproduced in a controlled layout, 100% repro):

• Windows Node archives put executables at the version dir root (version_bin_dir() returns the root for exactly this reason)
• the shim dispatcher's :resolve only checked %NVM_DIR%\%CURRENT%\bin\ — a directory that never exists on Windows
• PATH order is shims;active;bin, so the broken dispatcher always won resolution before the working active junction
• secondary bug: the shim set NVM_DIR=%USERPROFILE%\.nvm.rust unconditionally, clobbering custom NVM_DIR

Fix

• :resolve now checks the version dir root first (.exe then .cmd), keeping bin\ as fallback — applied to both copies of the script (src/shim.rs + install.ps1)
• NVM_DIR guard: if not defined NVM_DIR set ...
• quoted set "BIN=..." in install.ps1 (spaces in paths)

Tests

• src/shim.rs: root-before-bin ordering + NVM_DIR guard content tests
• tests/win_shim_dispatch.rs: install.ps1 content assertions + a Windows end-to-end test — real nvm refresh generates the shims against a root-layout fake version dir, then dispatching shims\node.cmd --version must succeed (this exact scenario failed before the fix)

Test plan

• cargo fmt --check / clippy -D warnings / cargo test --all green locally (18 groups)
• Manual repro confirmed broken before / fixed after
• CI green on Ubuntu / Windows / macOS