feat: migrate ~/.vite-plus into split VP_*/XDG dirs on vp upgrade

[forehalo]

Context

#2346 introduces split XDG/platform layout for fresh installs and keeps existing ~/.vite-plus installs on the legacy monolithic root (grandfathered). We intentionally did not ship automatic layout migration in that PR: moving a live global install is high risk (Windows file locks, PATH/profile cutover, concurrent shims).

Follow-up to #2346 / #827. Branch sketch: feat/layout-migrate (local experiment; re-land carefully).

Goal

On vp upgrade (and reinstall via installers where appropriate), detect a default legacy install at ~/.vite-plus / %USERPROFILE%\.vite-plus and split its contents into the correct category roots, then remove the legacy root.

From under ~/.vite-plus	To (Unix defaults)
version dirs, current, runtimes, packages, bins	VP_DATA_DIR / XDG_DATA_HOME/vite-plus / ~/.local/share/vite-plus
config.json	config dir (~/.config/vite-plus, …)
session / upgrade-check state	state dir
shims / env scripts	regenerate into bin + config (do not copy relative links / stale env)
resolve cache	drop (rebuild)

Custom VP_HOME roots stay out of auto-migrate (deprecated pin only).

Design constraints (from review)

• Copy-first, then delete legacy root (no long-lived tombstone unless Windows locks force a retry)
• Never delete legacy before split data/current is verified
• Conflict if split data already has a healthy unrelated install
• Shell profiles that source ~/.vite-plus/env* must be rewritten or cleaned
• N-1 users: best-effort upgrade path + install script as guaranteed fallback
• Full plan notes live in the dirs-path-resolution design discussion

Acceptance

• Machine with only ~/.vite-plus runs vp upgrade once → split roots populated, ~/.vite-plus gone, shims/env work after shell restart
• Fresh install never creates ~/.vite-plus
• CI covers legacy→split migrate + implode; unit tests for preflight/copy/delete
• No silent migrate of custom VP_HOME roots