feat(toolchain): add runtime capability registry

[szmyty]

Summary

Introduces the runtime tool/provider capability registry and reproducible selected-toolchain fingerprint required by #359.

This makes external tool availability, compatibility, provider identity, and capabilities first-class planner inputs instead of scattered executable assumptions.

Architecture

• Adds a canonical, versioned built-in registry in crates/renderflow-core/data/tool-registry.yaml.
• Adds the Rust ToolRegistry runtime model for built-in and runtime/plugin-provided tool capabilities.
• Uses the bounded process service from Centralize and harden external tool execution with bounded process policies #356 for executable/version probing.
• Preserves provider identity when transform definitions become graph edges so plans can explain which implementation was selected.
• Filters unavailable/incompatible providers during planning and retains explainable provider/fallback information.
• Computes a deterministic fingerprint from the providers relevant to the selected plan rather than the entire host environment.
• Carries the selected toolchain fingerprint into execution-plan evidence and artifact DAG cache compatibility.
• Registers dynamic YAML command transforms as runtime providers instead of pretending arbitrary executables are built-in tools.
• Reworks doctor and adds structured tools list, tools inspect, and capability surfaces backed by the same machine-readable registry model.
• Exposes the registry/fingerprint model through the SDK without exposing planner internals.
• Adds generated tool-registry documentation and makes docs CI verify that the generated projection is current.

Built-in capability model

Registry descriptors capture stable tool/provider IDs, executable/version probes, supported platforms/architectures, capability IDs, media constraints, determinism, local/network behavior, fidelity/loss characteristics, required environment/configuration, validation notes, licensing/distribution notes, and fallback/substitution relationships.

Compatibility and boundaries

• Flow remains responsible for cross-holon orchestration; Renderflow owns transform-provider discovery and transform planning.
• The artifact store remains responsible for payload identity/storage, not process/tool policy.
• Stabilize Transform v2 and the plugin SDK around typed artifact I/O #357 remains the place to stabilize the public Transform v2/plugin contract; Create a runtime tool capability registry and reproducible toolchain fingerprint #359 provides the capability-registration seam without prematurely freezing planner internals.
• Unknown dynamic command tools are runtime providers, not silently promoted into the canonical built-in catalog.

Tests

Coverage includes canonical registry parsing/validation, fake probe results, availability-state classification, version compatibility, stable tool/capability IDs, selected-toolchain fingerprint stability/change detection, provider-aware graph planning, runtime provider registration, registry-driven Tectonic availability checks, and the existing CLI/library/plugin example suite.

Final validation

Final review commit: 5f9e7a9865d321cc9fae869d5ff5b4afb64a0941 (single Conventional Commit).

All PR-triggered workflows passed on that exact SHA:

• CI 33442142287 — success
  • Clippy with warnings denied
  • full Rust workspace build
  • full Rust workspace tests
  • web formatting/lint/typecheck/tests/build
  • portable installer build/install smoke test
  • Arch Linux stable PKGBUILD validation
  • Arch Linux git PKGBUILD validation
  • Snap build, install, renderflow --version smoke test, and artifact upload
  • bundled hello-world/transforms dry-runs
  • library embedding example
  • plugin registration example
• docs 33442142327 — success
  • canonical tool-registry documentation generation
  • generated-doc zero-diff verification
  • MkDocs build
  • versioned docs build smoke test
• Commit Lint 33442142130 — success

Closes #359

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