perf(shim): optimize shim startup time and binary size

[CalvinAllen]

Optimize shim execution for faster command invocation.

Problem

The shim binary is invoked on every node, npm, python, pip command. Current issues:

• Binary size: ~10MB (same as main CLI)
• Heavy dependencies: Pulls in net/http, progressbar, color libraries not needed at runtime
• Expensive operations on every invocation:
  • Directory tree walking to find .dtvem/runtimes.json
  • JSON parsing of config files
  • Multiple os.Stat() calls per directory level
  • Runtime provider initialization

Performance Targets

• Shim overhead: < 10ms
• Binary size: < 5MB (ideally < 3MB)

Optimization Opportunities

Quick Wins

• Strip debug symbols in builds (-ldflags="-s -w") - reduces ~30%
• Cache os.UserHomeDir() - called multiple times
• Don't import ui package in shim - use plain fmt.Fprintf

Medium Effort

• Create shim-specific "lite" provider interface (only needs Name, Shims, ExecutablePath, IsInstalled, ShouldReshimAfter)
• Remove HTTP/download dependencies from shim build
• Cache resolved paths with mtime invalidation
• Lazy-load providers - don't register until needed

Architecture Options (for discussion)

• Build tags to exclude heavy code from shim
• Separate lite provider package
• Bake resolved paths at reshim time

Analysis

The shim imports full runtime providers which import:

• net/http (~2MB) - for ListAvailable()
• progressbar/v3 (~500KB) - via ui package
• fatih/color (~200KB) - for colored output
• download package - for installation

None of these are needed for shim's core job: resolve version → find executable → exec.

Related

Split from #19 to focus specifically on shim performance.