Expose stablecoin position health through stablecoin_module

[3esmit]

Goal

Expose a deterministic, read-only position-health quote through
stablecoin_module, using the collateralization invariant added by #336 and
live fee/redemption projections.

Scope

• Add a pure stablecoin_ffi quote operation that evaluates a decoded
  Position with the current stability-fee accumulator, redemption price, and
  minimum collateralization ratio.

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

• Reuse the Position and Vault derivation/validation from Expose stablecoin position resolution and decoding #325.

• Read ProtocolParameters for minimum_collateralization_ratio and reuse the
  current global projections from Expose current stablecoin global projections #326. Callers must not supply trusted
  protocol values.

• Preserve the exact feat(stablecoin): add collateralization-check helper #336 calculation:

  • nominal_debt = normalized_debt_amount * current_accumulator / FIXED_POINT_ONE
  • collateral_value = collateral_amount * FIXED_POINT_ONE^2
  • required_collateral_value = nominal_debt * current_redemption_price * minimum_collateralization_ratio
  • the Position is collateralized when collateral_value >= required_collateral_value

• Return Position/Vault IDs, source Position amounts, projected protocol
  operands, nominalDebt, collateralValue, requiredCollateralValue, and
  isCollateralized. Encode every integer as an exact decimal string.

• Treat zero debt as collateralized, matching feat(stablecoin): add collateralization-check helper #336.

• Use checked wide-integer arithmetic and return stable errors for overflow,
  missing accounts, malformed state, ownership/PDA mismatches, and projection
  failures.

• Share a non-panicking calculation/predicate with the program invariant where
  practical. Do not catch the assertion helper's panic as normal quote flow.

Acceptance criteria

• Quote results match the program invariant for zero debt, comfortable surplus,
  exact equality, one unit below the boundary, and accumulator-driven health
  deterioration.
• Fixed vectors pin rounding at the single nominal-debt division used by feat(stablecoin): add collateralization-check helper #336.
• Maximum-width request/state values cross the Rust/C++/Logos boundary without
  precision loss; checked U256 overflow becomes a stable error.
• Position lookup rejects malformed data, wrong program ownership, wrong PDA,
  and stored owner/nonce/vault mismatches.
• Protocol reads reject missing or malformed parameters, accumulator,
  redemption-state, and clock accounts.
• No floating-point values appear in request or response contracts.
• Relevant Rust/module tests and nix build .#stablecoin-module pass.