Scaffold stablecoin program workspace structure

[0x-r4bbit]

Create the workspace layout mirroring existing programs:

• stablecoin/core/ — shared types and instruction enum
• stablecoin/src/ — program logic
• stablecoin/methods/ — host crate with build.rs
• stablecoin/methods/guest/ — guest binary (separate workspace, RISC-V target)

Add stablecoin to the root Cargo.toml workspace members.

Also, ensure its tests are executed on CI.