Expose stablecoin openPosition transaction through stablecoin_module

[3esmit]

Goal

Expose the fee-aware open_position instruction rebuilt by #341 as an
openPosition transaction method in stablecoin_module.

Scope

• Add a typed stablecoin_ffi planner for
  Instruction::OpenPosition { position_nonce, initial_collateral_amount }.

• Add openPosition(request) to StablecoinModuleImpl, accepting:

  • ownerId
  • exact decimal-string positionNonce
  • exact decimal-string initialCollateralAmount
  • userCollateralHoldingId

• Derive Position and Vault PDAs internally from the stablecoin program ID,
  owner, and nonce, reusing Expose stablecoin position resolution and decoding #325.

• Derive/read ProtocolParameters, obtain its configured collateral definition,
  and supply canonical CLOCK_01 internally.

• Preserve the seven IDL accounts in exact order:

  1. owner
  2. Position
  3. Vault
  4. user collateral holding
  5. collateral definition
  6. protocol parameters
  7. CLOCK_01

• Preserve all IDL signer/writable/init metadata. Owner and user collateral
  holding are the two signers.

• Verify both signer accounts are public and controlled by the active wallet
  before submission.

• Preflight protocol initialization/frozen state, configured collateral
  definition, source-holding definition/program ownership, and absence of
  initialized Position/Vault state when the wallet/indexer surface permits it.

• Submit through the shared transaction path introduced for Expose permissionless stablecoin poke transactions #328 and return the
  same stable {status,error,transactionId} result shape.

• Reject fractional, negative, floating-point, malformed, and out-of-range
  nonce/amount inputs without submitting.

Acceptance criteria

• Planner output decodes exactly to
  Instruction::OpenPosition { position_nonce, initial_collateral_amount }.
• Fixed-vector tests pin Position/Vault derivation, seven-account order,
  signer/writable/init flags, program ID, and instruction encoding.
• Tests prove both required signer accounts are authorized and use the correct
  wallet nonces.
• Tests cover missing/malformed protocol parameters, frozen protocol, wrong
  configured collateral definition, wrong source-holding definition/program,
  existing Position/Vault state, missing signer control, transport failure,
  transaction rejection, and successful transaction ID mapping.
• Full-range u64 nonce and u128 collateral values cross the
  Rust/C++/Logos boundary without precision loss.
• No legacy five-account or collateral_amount encoding remains in the new
  planner or public method.
• Relevant Rust/module tests, module inspection, headless logoscore smoke
  tests, and nix build .#stablecoin-module pass.