refactor(runtimes): split provider files to slim shim binary

[CalvinAllen]

Summary

Splits each runtime provider (node, python, ruby) into a "shim half" (lite methods only) and a "full half" tagged //go:build !shim. Build the shim with -tags shim and the linker drops every heavy transitive dependency the shim never actually used.

Measured impact

Binary	main (#264)	This PR	Δ
dtvem-shim.exe	5.70 MB	2.68 MB	−3.0 MB (−53%)
dtvem.exe	10.94 MB	10.94 MB	unchanged

Combined with the -trimpath work in #264, the shim is now 2.68 MB versus 8.20 MB on f1dfab5 (pre‑#264) — a −67% total reduction since this slimming effort started.

Symbol audit — what's gone

go tool nm on the new shim no longer contains any of:

• github.com/bodgit/sevenzip (was 173 symbols)
• github.com/klauspost/compress (was 235)
• net/http (was 114)
• github.com/ulikunitz/xz (was 102)
• github.com/pierrec/lz4 (was 93)
• github.com/andybalholm/brotli (was 74)
• compress/bzip (was 50)
• archive/zip / archive/tar / compress/gzip (was 34 combined)
• crypto/tls (was 22)
• github.com/spf13/afero (was 10)
• github.com/hashicorp/golang-lru (was 10)
• github.com/schollz/progressbar (was 3)
• github.com/bodgit/plumbing (was 3)
• The 1.3 MB of embedded manifest JSON (internal/manifest/data/*.json)
• The Node.js release schedule (runtimes/node/data/schedule.json)
• The testing package (transitively via provider_test_harness.go)

What remains is encoding/json (for the shim‑map and runtimes.json), fatih/color + mattn/go-isatty + mattn/go-colorable (the user‑facing error output), and dtvem's own code.

Approach

One change to interface shape, the rest is a mechanical file split.

• internal/runtime/provider.go — keeps ShimProvider.
• internal/runtime/provider_full.go (//go:build !shim) — defines the full Provider interface with all heavy methods.
• internal/runtime/provider_shim.go (//go:build shim) — type Provider = ShimProvider (alias).

Because Provider always exists as a public type, the registry and its callers (including internal/shim/manager.go and every cmd/*.go file) need no changes. In default builds Provider is the full interface; in shim builds it collapses to ShimProvider and the heavy method calls on concrete types are gone — so klauspost/compress, sevenzip, et al. are no longer reachable.

Per‑runtime split (src/runtimes/{node,python,ruby}/):

• provider.go (untagged) — Name, DisplayName, Shims, ExecutablePath, IsInstalled, InstallPath, ShouldReshimAfter, GetEnvironment, init(). Imports stay minimal: fmt, os, filepath, runtime, internal/config, internal/constants, internal/runtime.
• provider_full.go (//go:build !shim) — Install, Uninstall, ListInstalled, ListAvailable, DetectInstalled, GlobalPackages, InstallGlobalPackages, ManualPackageInstallCommand, all version setters/getters, all install helpers. Heavy imports (internal/download, internal/manifest, internal/shim, internal/ui, encoding/json, os/exec, etc.) live only here.

Extras tagged !shim:

• internal/runtime/provider_test_harness.go — uses full Provider methods on a concrete value; only consumed by tests (default build).
• runtimes/node/lifecycle.go — embeds data/schedule.json and is only used by list-all.

Build config: -tags shim added to the build-shim step in rnr.yaml, .github/workflows/build.yml, and .github/workflows/release.yml. Main CLI build is untagged.

Test plan

• ./rnr.cmd check passes (format, lint, full test suite — every package green)
• go build ./... succeeds under the default build
• go build -tags shim ./src/cmd/shim succeeds
• Symbol audit (go tool nm) confirms the heavy packages are no longer present
• Main CLI binary size unchanged (10.94 MB)
• CI passes on all five matrix platforms
• Manual smoke test post‑merge: install a runtime, run python --version / node --version / ruby --version through the shimmed executables, verify "no version configured" fallback still works

Risk

Mechanical refactor. Watch points:

• Anything outside cmd/shim that calls full‑Provider methods needs to be in !shim files so the shim build compiles. Validated by build success on both tag configurations; the only file that needed tagging was provider_test_harness.go (and node/lifecycle.go, which is already tagged for the secondary win of dropping the embedded schedule).
• Concrete provider types in shim builds satisfy ShimProvider (== Provider in that build). In default builds they satisfy the full Provider. Verified by go vet + go test.

Resolves #265

[CalvinAllen]

/release-note Shim binary is now 67% smaller (8.2 MB → 2.7 MB), reducing disk footprint and cold-start cost for every shimmed command!