Expose stability fee accumulator state through stablecoin_module

[3esmit]

Goal

Expose the singleton StabilityFeeAccumulator account through
stablecoin_module, using the account type and PDA contract introduced by
#302.

Scope

• Add a pure stablecoin_ffi operation that derives the accumulator PDA and
  exactly decodes stablecoin_core::StabilityFeeAccumulator from a validated
  public-account read.
• Add stabilityFeeAccumulator() to StablecoinModuleImpl. It must derive and
  read the singleton account; callers must not supply an arbitrary account ID.
• Return a stable map containing both base58 and hex account IDs,
  accumulatedRateAtLastAccrual, and lastAccruedAt.
• Return all integer fields as exact decimal strings.
• Reject an absent account, wrong PDA, wrong program owner, malformed/truncated
  data, trailing bytes, or a backend read failure with stable error codes.
• Keep projection to the current clock time out of this raw-state operation;
  that belongs to the current-global-projections sub-issue sourced from feat(stablecoin): add current value projection helpers #311.

Acceptance criteria

• PDA derivation matches
  compute_stability_fee_accumulator_pda(stablecoin_program_id) for fixed
  vectors and the configured deployed program.
• Successful reads preserve full-range u128 and u64 values without
  floating-point conversion.
• Decoder tests cover valid state, uninitialized state, wrong owner, wrong PDA,
  truncated data, trailing data, and maximum 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.