Expose current stablecoin global projections

[3esmit]

Goal

Expose current, clock-projected stablecoin global values through
stablecoin_module, using the projection helpers merged in #311.

Scope

• Add a pure stablecoin_ffi projection operation backed directly by
  compute_current_accumulated_rate and
  compute_current_redemption_price.
• Add currentGlobalState() to StablecoinModuleImpl.
• Read and validate ProtocolParameters, StabilityFeeAccumulator,
  RedemptionPriceState, and the canonical CLOCK_01 account through
  logos_execution_zone.
• Use the clock account's Unix-millisecond timestamp. Do not accept a
  caller-supplied wall clock for the live operation.
• Return raw anchors and timestamps together with currentAccumulatedRate,
  currentRedemptionPrice, and projectedAt, all as exact decimal strings.
• Preserve the on-chain semantics: saturating timestamp subtraction and the
  MAXIMUM_COMPOUNDING_WINDOW_MILLISECONDS seven-day clamp.
• Surface invalid global ownership/PDA/data and invalid clock reads through
  stable error codes.

Acceptance criteria

• Projection output matches the Rust helpers byte-for-byte for identity,
  growth, decay, zero elapsed time, inverted timestamps, exactly-at-clamp, and
  beyond-clamp cases.
• The live module method reads each required singleton and the canonical clock;
  it never projects from process wall time.
• Maximum fixed-point values supported by the helpers remain exact across the
  API boundary; JSON floats are rejected.
• Tests prove now < last_updated_at and now < last_accrued_at collapse to
  the stored anchors without underflow.
• nix build .#stablecoin-module and relevant Rust/module tests pass.

Dependencies

Depends on the scaffold, raw accumulator-state, and raw
redemption-price-state sub-issues under #321.