feat(dsl): chainable .setup() on all toolchains (Python + TypeScript)

[markovejnovic]

What

Adds a chainable .setup(cmd, *, cwd, label, cache, env) method to every install-bearing toolchain object in both DSLs, so users can splice a prep step (codegen, fixtures, extra tooling) between "toolchain installed" and the action leaves. Each call returns a new toolchain of the same type with its install cursor advanced; later actions (compile/test/build/…) fork from it.

hm.elixir(path="elixir").setup("mix proto.gen").compile()   # chainable, immutable

This replaces the dataclasses.replace(installed=…) internal hack consumers were forced into.

Coverage

All 8 install-bearing classes, per language:
ElixirProject, PythonToolchain, GoToolchain, JsProject, ZigProject, ZigToolchain, RustToolchain, CMakeToolchain.

• Python: one shared advance_install() helper (dataclasses.replace preserves every other field); read-only _HasInstalled Protocol member so frozen dataclasses satisfy it.
• TypeScript: per-class setup() reconstructing via the class constructor; JsProject now retains pm to rebuild faithfully.

RustProject/CMakeProject (warmup/built wrappers) are intentionally excluded — their install boundary is on the toolchain (the correct pre-warmup insertion point); use hm.rust.toolchain().setup(...).project(...).

Tests / gates

• Python: ruff clean, ty clean, full suite 563 passed (+ new parametrized tests/test_setup.py, 9 cases).
• TypeScript: tsc --noEmit clean, full suite 421 passed (+ new tests/setup.test.ts, 10 cases).

Follow-ups

• DSL reference docs regenerate from the new docstrings at deploy (make docs-generate).
• A new hm release is needed before consumers (e.g. simci's .hm/pipeline.py) can use .setup().