feat: Unified Tool Registry & Download System

[avrabe]

Summary

Consolidate scattered toolchain download logic into a single unified tool registry system.

Current State

• Each toolchain .bzl file has its own download logic
• Platform detection duplicated across files
• Version management scattered
• ~500 lines of duplicate download code

Proposed Solution

Single API for all tool downloads:

tool_registry.download(ctx, "wasm-tools", "1.240.0", platform)

Key Changes

1. Central tool_registry.bzl module that:

   • Reads all checksums/tools/*.json at repository rule time
   • Provides unified download API
   • Handles platform detection ONCE, reuses everywhere
   • Central cache management
   • Automatic version resolution with "latest" support

2. Refactor existing toolchains to use registry API

Benefits

• DRY: Remove ~500 lines of duplicate download code
• Single point for security auditing
• Easier version bumps (one JSON file per tool)
• Consistent error handling

New Features Enabled

• bazel run @rules_wasm_component//tools:update_checksums - auto-fetch latest versions
• Version compatibility matrix validation
• Offline mode with pre-populated cache

Impact

• Effort: Low
• Risk: Low
• Value: Medium

Related

• Builds on existing checksums/tools/*.json infrastructure
• Foundation for Version Bundles (#TBD)

[avrabe]

Completed in PR #287. JSON files in checksums/tools/*.json are now the single source of truth - eliminated 860 lines of fallback data from registry.bzl.