Refactor BB2/Sandbox code into package extension

This refactors all BinaryBuilder2 and Sandbox-related code into a
package extension (SandboxExt) that loads conditionally when Sandbox
and BinaryBuilder2 are available. This significantly reduces the
dependency footprint for users who don't need sandboxing.

Changes:
- Move all BB2/Sandbox dependencies from [deps] to [weakdeps]
- Keep Scratch as primary dependency (used by base package)
- Create ext/SandboxExt.jl extension triggered by Sandbox + BinaryBuilder2
- Move all sandbox functions to ext/sandbox_impl.jl
- Add sandbox_stubs.jl with 0-method function declarations
- Use invokelatest for all calls to extension functions
- Automatically load Sandbox/BB2 when sandbox mode requested
- Access BinaryBuilderToolchains through BinaryBuilder2 import
- Fix nested module references (e.g., BinaryBuilder2.BinaryBuilderSources)

Extension loading:
- Extension triggers on: Sandbox + BinaryBuilder2
- BinaryBuilderToolchains accessed via BinaryBuilder2, not as trigger
- has_sandbox_support() implemented by extension to return true
- Use hasmethod() check before attempting to load
- Use Base.invokelatest() for all extension function calls

Error handling:
- Distinguish between "not installed" vs other load errors
- Provide helpful error messages with --direct flag suggestion
- Show full backtrace in verbose mode for debugging

This maintains full backward compatibility while making the package
much lighter for users who run with --direct mode.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Keno