feat(stablecoin): expose stability fee accumulator

[3esmit]

Summary

• add a pure decoder that derives and validates the stability fee accumulator PDA, owner, and exact account data
• expose no-argument stabilityFeeAccumulator() with base58/hex IDs and exact decimal integer strings
• add stable error mapping plus Rust boundary tests and C++ one-read response tests

Scope

Returns the stored accumulator snapshot only. No clock projection, redemption state, position, or quote behavior is included.

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
• make clippy-guest
• make build-programs
• committed IDL drift check
• nix build path:.#stablecoin_ffi path:.#stablecoin-module
• nix build path:.#stablecoin-module-tests

Closes #323

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