feat(stablecoin): expose position account

[3esmit]

Summary

• add pure stablecoin_ffi operations for deterministic position/vault PDA resolution and strict Position decoding
• expose positionAccount with exact owner/nonce lookup, one direct account read, and stable not_found responses carrying derived IDs
• preserve full-range integer values as decimal strings and validate program ownership plus all stored position identity fields
• add fixed PDA vectors, malformed/invariant rejection coverage, C boundary tests, and universal module tests

Validation

• cargo +nightly fmt --all -- --check
• taplo fmt --check .
• make clippy
• 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
• IDL drift check
• make build-programs
• nix build path:.#stablecoin_ffi path:.#stablecoin-module path:.#stablecoin-module-tests

Closes #325

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