feat(stablecoin): expose current global state

[3esmit]

Summary

• Add a pure stablecoin_ffi projection over validated protocol parameters, accumulator state, redemption state, and canonical CLOCK_01.
• Expose currentGlobalState() with four public account reads and exact decimal-string output.
• Preserve saturating elapsed-time handling and the seven-day compounding clamp by calling the stablecoin core helpers directly.

Scope

Read-only global projection only. No controller quote, poke submission, position logic, guest instruction, program-core, or IDL changes.

Stack

Built on #333 because this API composes its accumulator and redemption-state surfaces.

Validation

• cargo +nightly fmt --all -- --check
• taplo fmt --check .
• RISC0_SKIP_BUILD=1 cargo +1.94.0 clippy --workspace --all-targets -- -D warnings
• make clippy-guest
• 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
• nix build path:.#stablecoin_ffi path:.#stablecoin-module path:.#stablecoin-module-tests
• make build-programs
• Tracked program IDLs match committed artifacts.

Closes #326

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