feat: add packaged reusable build-step providers

[wolfv]

Stack

2 of 4 in the experimental build-steps series.

• Previous: feat: add named and local reusable build steps #2763 — named and local reusable steps
• This PR: feat: add packaged reusable build-step providers #2764 — packaged providers
• Next: feat: cache steps and export post-build metadata #2765 — caching and post-build metadata
• Top: feat: generate recipes with pre-solve metadata backends #2767 — pre-solve metadata backends

Summary

Build on named steps by allowing reusable pipelines to ship in conda packages.

• resolve provider:step@version references before the final build/host solve
• install providers into isolated, content-addressed prefixes without running link scripts
• support typed with inputs, defaults, selectors, and nested provider pipelines
• collect provider build/host requirements into the recipe solve
• record exact provider provenance and deterministic content/rendered fingerprints in package identity
• sanitize provider channel provenance and reject late variant dependencies that missed initial expansion
• reject unsafe nesting, malformed provider DAGs, inheritance changes, and unsupported multi-output use
• allow providers to emit license-file metadata

Examples

Add packaged CMake, Meson, Go, and Rust provider examples plus local provider/consumer fixtures.

Validation

• cargo check --all-targets
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all --check
• git diff --check
• 4 layer-specific build-step/provider E2E tests

The complete cross-platform end-to-end coverage additionally runs on the top PR, #2767.