remove load_or_install, centralise install_root / bin_dir handling, cleanup nested provider handling

Author: pirate

README.md

@@ -654,7 +654,7 @@ provider = PipProvider(install_root=Path("/tmp/venv")).get_provider_with_overrid
 
 Providers with isolated install locations also expose a shared constructor surface:
 
-- `install_root`: shared alias for provider-specific roots such as `pip_venv`, `npm_prefix`, `cargo_root`, `gem_home`, `gopath`, `nix_profile`, `docker_shim_dir.parent`, and `brew_prefix`.
+- `install_root`: shared alias for provider-specific roots such as `pip_venv`, `npm_prefix`, `cargo_root`, `gem_home`, `gopath`, `nix_profile`, `docker_root`, and `brew_prefix`.
 - `bin_dir`: shared alias for providers that separate package state from executable output, such as `gem_bindir`, `gobin`, and `docker_shim_dir`.
 - `provider.install_root` / `provider.bin_dir`: normalized computed properties you can inspect after construction, regardless of which provider-specific args were used.
 - Legacy provider-specific args still work. The shared aliases are additive, not replacements.

docs/generate.py

@@ -462,7 +462,7 @@
     "docker": [
         {
             "name": "ABX_PKG_DOCKER_ROOT",
-            "description": "Overrides docker_shim_dir parent.",
+            "description": "Overrides docker_root / install_root.",
         },
     ],
     "brew": [