test(stablecoin): add stateful user journey coverage

[3esmit]

Problem

The stablecoin module stack exposes global-state reads, redemption-rate quotes, and permissionless refresh plans. Existing tests cover individual API mappings, but do not exercise the account state transitions a user or keeper sees across several calls.

This PR adds stateful user-history coverage. Each journey keeps protocol parameters, the stability-fee accumulator, redemption-price state, oracle data, and clock data in memory. It builds a transaction plan through the public FFI API, decodes the serialized instruction, executes the native stablecoin handler, and feeds the post-state into the next read.

Journeys

• Startup with missing global accounts, followed by initialized decoders, projections, and a ready quote.
• Fee projection followed by accumulator-only persistence and a same-time no-op repeat.
• Successive controller quotes and updates, including persisted controller state and equivalence between combined refresh and standalone operations.
• Recovery from stale-oracle, zero-price, and too-soon blockers through fee-only refreshes, repair, and retry, including exact boundary checks.
• Rechecking a previously ready quote after another keeper changes state and the oracle becomes stale, then completing a fresh retry.
• Recovery from malformed callers, missing accounts, wrong owners, and malformed account bytes without mutating state, followed by a valid execution.
• Long idle periods across the compounding clamp, refresh after idle, and exact u128/i128 boundary serialization.

Implementation

• Added modules/stablecoin/ffi/src/api/journeys.rs with seven end-to-end user-history tests at the FFI/native-program seam.
• Added the native stablecoin program as a test-only dependency of stablecoin_ffi.
• Added an optional C++ module journey target linked to the production stablecoin_ffi library; the existing mock-based module suite remains unchanged.
• The production-FFI journey drives real module reads, projections, quotes, serialized submissions, and rereads through the account store.
• Preserved one commit per journey:
  • 078f5b6c startup global-state journey
  • 31406961 fee accrual journey
  • 3c836760 controller update journey
  • 8c74fed5 blocked update recovery
  • 2a5a784f stale quote retry
  • 2bccf9b2 account recovery journey
  • 39ccef34 long idle journey

Stacked on #340, following #339, #338, #333, and #332.

Validation

• RISC0_DEV_MODE=1 cargo +1.94.0 test --workspace --exclude integration_tests
• RISC0_DEV_MODE=1 cargo +1.94.0 test -p integration_tests
• RISC0_DEV_MODE=1 cargo +1.94.0 test -p stablecoin_ffi
• RISC0_DEV_MODE=1 cargo +1.94.0 test -p stablecoin_program
• RISC0_SKIP_BUILD=1 cargo +1.94.0 clippy --workspace --all-targets -- -D warnings
• cargo +nightly fmt --all -- --check
• taplo fmt --check .
• Tracked guest IDL regeneration and diff
• nix build path:.#stablecoin-module-tests --no-link -L
• nix build path:.#stablecoin-module --no-link -L

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