[v1] Build-flag split for experimental node subsystems

[SMC17]

Problem
The v1 release needs this work to make the toolkit claim defensible.

Scope

• Separate experimental node-style modules from the default release path.
• Ensure the default build produces a toolkit-first artifact layout.
• Keep experimental surfaces available only behind explicit flags.

Files

• build.zig
• src/main.zig
• src/modules.zig

Gates

• Gate A

Acceptance Criteria

• Experimental subsystems are behind build flags or excluded from the release target.
• Default build path matches v1 toolkit positioning.
• Examples and docs reference the new default entrypoints.

Related

• Epic: [Epic] v1 XRPL Toolkit release #45
• Milestone: v1 XRPL Toolkit