feat(stablecoin): add Logos API module

[3esmit]

Summary

• add the universal stablecoin_module core module and Rust stablecoin_ffi boundary
• expose program discovery, exact ProtocolParameters reads, and initializeProgram orchestration
• derive and preflight all singleton PDAs, preserve exact numeric values, and submit the nine-account plan with only the admin signer
• add root Nix outputs, module packaging, Rust coverage, and Logos module tests

Validation

• cargo +nightly fmt --all -- --check
• taplo fmt --check .
• RISC0_SKIP_BUILD=1 cargo +1.94.0 clippy --workspace --all-targets -- -D warnings
• RISC0_DEV_MODE=1 cargo +1.94.0 test --workspace --exclude integration_tests
• RISC0_DEV_MODE=1 cargo +1.94.0 test -p integration_tests
• tracked guest IDL drift check
• nix build path:.#stablecoin-module -L
• nix build path:.#stablecoin-module-tests --no-link -L
• module inspection and headless logoscore programInfo smoke test

Closes #322

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[0x-r4bbit]

#330 supersedes this.