perf(shim): add shim-to-runtime mapping cache for O(1) lookups

[CalvinAllen]

Summary

• Add persistent shim-to-runtime mapping cache (~/.dtvem/cache/shim-map.json)
• Generate cache during reshim operations with runtime associations
• Enable O(1) lookups in mapShimToRuntime() using cached map
• Fix dynamic shim resolution for globally installed packages (tsc, eslint, black, etc.)

Problem

Previously, mapShimToRuntime() only knew about core provider shims (node, npm, python, pip, etc.). Dynamically installed packages like tsc (from TypeScript) or black (from pip) would fail with "runtime provider not found" because the fallback returned the shim name as the runtime name.

Solution

During reshim, we already scan each runtime's directories to find executables. Now we also build a mapping of shim → runtime and save it to ~/.dtvem/cache/shim-map.json:

{
  "node": "node",
  "npm": "node",
  "tsc": "node",
  "python": "python",
  "black": "python"
}

The shim executable loads this cache once (using sync.Once) for O(1) lookups.

Test plan

• Unit tests for cache save/load functionality
• Unit tests for LookupRuntime with cache hit/miss scenarios
• Unit tests for sync.Once behavior (cache only loads once)
• Manual testing: reshim generates cache file
• Manual testing: core shims work (node, npm)
• Manual testing: dynamic shims work (corepack)

Closes #93