Auto-install: toolchain-gated LSP servers (runtimeInstall + canonical bin discovery)

[apmantza]

Problem

ensureTool() can only auto-install servers present in the installer TOOLS registry, which covers npm/pip/gem + single-binary github/maven/archive. A family of language servers is not in the registry because they require a full language toolchain or a non-binary install, so ensureTool returns UNAVAILABLE and they only run if the user has the binary on PATH:

• gopls — go install golang.org/x/tools/gopls@latest (needs Go; lands in GOPATH/bin)
• csharp-ls / fsautocomplete — dotnet tool install (needs .NET; lands in ~/.dotnet/tools)
• sourcekit-lsp — ships with the Swift toolchain (not separately installable)
• haskell-language-server — ghcup-managed
• ocamllsp — opam-managed
• nixd — nix-managed

These are deliberately out of scope today ("bring your own server"). This issue tracks closing that gap properly.

Proposal

1. runtimeInstall step gated on toolchain presence. When the language runtime is on PATH (e.g. go, dotnet, opam), run the canonical install command; otherwise report unavailable (never auto-install the toolchain itself). The resolveAndLaunch spec already has a runtimeInstall hook — extend coverage to these servers.
2. Canonical-bin discovery. Teach installer discovery to look in toolchain-managed bin dirs even when not on PATH: GOPATH/bin (or go env GOPATH), ~/.dotnet/tools, ~/.cargo/bin, opam/ghcup switch bins. This is the real fix for "the toolchain is installed but its server bin isn't on PATH" — discovery, not user PATH edits.

Out of scope / related

• Whole-tree GitHub-release servers that need their full distribution (clangd's builtin headers, lua-language-server's lib/, kotlin-language-server/elixir-ls JVM/BEAM) need a platform-matched archive strategy (current ArchiveSpec has a single hardcoded URL). Track separately if desired.
• clojure-lsp + gleam (single native binaries) are now registered (f263cf3).

Context

Surfaced characterizing the LSP capability matrix (#240) — the nightly tool-smoke only ⚠-skips these servers, so they're never exercised.

[apmantza]

Progress (1efc4e0): clangd now auto-installs instead of being PATH-only.

Built the reusable platform-matched archive URL machinery this whole family needs: ArchiveSpec.url now accepts a (platform, arch) => url | undefined resolver (plain string still works), and resolveArchiveUrl degrades to "unavailable" when a platform has no build. clangd ships as a self-contained archive tree bundle (stripComponents:1 drops the clangd_<ver>/ wrapper, treeMarker: "bin", no launcher), and a new resolveAndLaunchTreeBinary helper resolves PATH-first (system clangd wins) then the managed bundle, launching <bundle>/bin/clangd directly — no external runtime. Verified end-to-end on win32 (download → extract → handshake).

Remaining under this issue:

• Other archive-tree servers on the now-existing shape: lua-language-server (same self-contained bin-in-tree shape as clangd; also exercises per-arch matching), then kotlin-language-server / elixir-ls (launcher-script-in-tree needing an ambient JVM/BEAM toolchain).
• The core proposal — runtimeInstall + canonical-bin discovery for the toolchain-gated servers (gopls/csharp-ls/fsautocomplete/sourcekit-lsp/haskell-language-server/ocamllsp/nixd) — still open.