refactor(shim): implement lite provider interface

[CalvinAllen]

Refactor the shim to use a lightweight provider interface that excludes heavy dependencies.

Problem

The shim imports full runtime providers which pull in dependencies not needed at runtime:

• net/http (~2MB) - used only by ListAvailable()
• download package - used only by Install()
• progressbar/v3 (~500KB) - used by download
• Detection code for nvm/pyenv/fnm - used only by DetectInstalled()

Shim's Actual Needs

The shim only uses these provider methods:

type ShimProvider interface {
    Name() string
    DisplayName() string
    Shims() []string
    ExecutablePath(version string) (string, error)
    IsInstalled(version string) (bool, error)
    ShouldReshimAfter(shimName string, args []string) bool
}

It does not need:

• Install() / Uninstall()
• ListAvailable() / ListInstalled()
• DetectInstalled()
• GlobalPackages() / InstallGlobalPackages()

Proposed Solutions

Option A: Build Tags

Use //go:build !shim to exclude heavy methods from shim builds:

//go:build !shim

func (p *Provider) ListAvailable() ([]runtime.AvailableVersion, error) {
    // HTTP code here
}

Pros: Minimal code changes, single provider file
Cons: Build complexity, easy to forget tags

Option B: Interface Split

Create separate ShimProvider and FullProvider interfaces:

src/internal/runtime/
├── provider.go      # Full interface
└── shim_provider.go # Lite interface for shim

src/runtimes/node/
├── provider.go      # Core methods (both interfaces)
├── install.go       # Install/download (full only)
└── detect.go        # Detection code (full only)

Pros: Clean separation, explicit dependencies
Cons: More files, some duplication

Option C: Lazy Loading

Only import heavy packages when methods are called (using plugin or reflection).

Pros: No build changes
Cons: Complex, runtime overhead, not idiomatic Go

Expected Impact

• Shim binary size: ~7.2MB → ~3-4MB (after symbol stripping)
• Removes net/http initialization overhead
• Faster cold starts

Related

• perf(shim): optimize shim startup time and binary size #72 - Parent issue for shim optimization
• perf(ci): strip debug symbols from builds #73 - Symbol stripping (first optimization)