Expose stablecoin position resolution and decoding

[3esmit]

Goal

Expose deterministic position lookup and exact decoding for the current
Position shape introduced by #304.

Scope

• Add pure stablecoin_ffi operations for position and vault PDA derivation
  from (owner_account_id, position_nonce) and for exact Position decoding.

• Add positionAccount(request) to StablecoinModuleImpl, accepting
  ownerId and exact decimal-string positionNonce.

• Derive the position and vault IDs internally, read the position account, and
  validate:

  • stablecoin program ownership and expected position PDA
  • stored owner and nonce equal the request
  • stored vault equals the expected vault PDA

• Return derived IDs in base58 and hex plus collateralAmount,
  normalizedDebtAmount, and openedAt as exact decimal strings.

• Return the derived position/vault IDs with a stable not_found result when
  that owner/nonce has no position, allowing clients to distinguish absence
  from malformed state.

• Do not claim global position enumeration. The on-chain program has no index;
  callers must know or manage owner nonces.

Acceptance criteria

• Fixed-vector tests pin both PDA derivations and their domain separation.
• Exact decoding rejects wrong owner, wrong program, mismatched stored owner,
  nonce, or vault, malformed/truncated data, and trailing bytes.
• Full-range collateral, normalized debt, nonce, and timestamp values cross the
  Rust/C++/Logos boundary without precision loss.
• Module tests cover valid, absent, malformed, and invariant-mismatch reads.
• Public response/error maps remain stable and machine-readable.
• nix build .#stablecoin-module and relevant Rust/module tests pass.

Dependencies

Depends on the stablecoin_module scaffold and initializeProgram sub-issue
under #321.

Out of scope

Submitting open_position, withdraw_collateral, or repay_debt; global
position indexing; UI work.