feat(stablecoin): expose redemption price state

[3esmit]

Summary

• add pure stablecoin_ffi decoding for the singleton RedemptionPriceState account
• add redemptionPriceState() with one derived public-account read and stable error mapping
• preserve account IDs and all u128, signed i128, and u64 values as exact strings
• document raw-state semantics without current-price projection or controller simulation

Validation

• fixed PDA vector and signed/boundary integer decoder tests
• absent, backend-error, wrong-PDA, wrong-owner, truncated, and trailing-data tests
• universal module success/error contract tests
• Rust format, TOML format, workspace Clippy, unit tests, integration tests, guest Clippy, guest builds, and tracked IDL checks
• stablecoin_ffi, stablecoin-module, and stablecoin-module-tests Nix builds

Stack

Targets #332 to reuse its shared module-test harness. This change has no functional dependency on the accumulator API and should be retargeted to main after #332 merges.

Closes #324

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