Expose redemption price state through stablecoin_module

[3esmit]

Goal

Expose the singleton RedemptionPriceState account through
stablecoin_module, using the signed controller state and PDA contract added
by #303.

Scope

• Add a pure stablecoin_ffi operation that derives the redemption-price-state
  PDA and exactly decodes stablecoin_core::RedemptionPriceState from a
  validated public-account read.
• Add redemptionPriceState() to StablecoinModuleImpl. It must derive and
  read the singleton account rather than accept an arbitrary account ID.
• Return both account-ID encodings plus
  redemptionPriceAtLastUpdate, redemptionRatePerMillisecond,
  controllerIntegralTerm, and lastUpdatedAt.
• Preserve u128, signed i128, and u64 values as exact decimal strings.
• Reject an absent account, wrong PDA, wrong program owner, malformed/truncated
  data, trailing bytes, or backend read failure with stable error codes.
• Keep current-price projection and controller simulation out of this raw-state
  read; those are covered by the feat(stablecoin): add current value projection helpers #311 and feat(stablecoin): add redemption rate controller + clamp constants #312 API sub-issues.

Acceptance criteria

• PDA derivation matches
  compute_redemption_price_state_pda(stablecoin_program_id) for fixed vectors
  and the configured deployed program.
• Positive, zero, negative, minimum, and maximum controllerIntegralTerm
  values round-trip without sign or precision loss.
• Decoder tests cover valid state, uninitialized state, wrong owner, wrong PDA,
  truncated data, trailing data, and boundary integer values.
• Module tests prove one live read of the derived account and stable public
  response/error shapes.
• nix build .#stablecoin-module and relevant Rust/module tests pass.

Dependencies

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