test(stablecoin): add the full position lifecycle integration test

[gravityblast]

Adds stablecoin_full_position_lifecycle, the end-to-end test that closes out
Plan 3: open → deposit → borrow → accrue → repay → withdraw → close, all through
the zkVM against a really-bootstrapped protocol (initialize_program, not
force-inserted globals).

This is "Mode A" from the issue — Plan 2 has shipped, so accrue_stability_fee
runs between borrowing and repaying. The test asserts the accumulator moves above
FIXED_POINT_ONE and that the accrued nominal debt exceeds the amount minted,
which is the only place §6.3's directional rounding gets exercised at a
non-trivial accumulator.

That fee gap is why the owner is seeded with a stablecoin balance before
borrowing: repaying in full costs more than was minted, so the shortfall has to
come from somewhere — the market, per §2. total_supply is credited to match, or
the chained Burn underflows.

Two things the run surfaced, both already handled in the stack:

• The bootstrap fixture's oracle is anchored at timestamp 0, which generate_debt's
  staleness gate correctly rejects against a real wall clock. The test seeds a
  fresh observation.
• close_position cannot clear the account to Account::default(); see feat(stablecoin): add close_position instruction #354.

assert_fungible_balance was hardcoded to the collateral definition, so it grew
an assert_fungible_balance_of variant for the stablecoin holding.

Last of eight issues in Plan 3 (#173). Stacked on #354.

closes #181

[Copilot]

🟡 Changes recommended

The new lifecycle test mutates stablecoin total_supply for the seeded buffer but doesn’t assert post-borrow/repay supply and balances stay consistent, leaving a key end-to-end invariant unverified.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds a new stablecoin end-to-end integration test that exercises the full “Plan 3” position lifecycle through the zkVM on top of a properly bootstrapped protocol state (via initialize_program), including Plan 2 stability-fee accrual between borrow and repay.

Changes:

• Add stablecoin_full_position_lifecycle integration test covering open → deposit → borrow → accrue → repay → withdraw → close.
• Generalize assert_fungible_balance by introducing assert_fungible_balance_of to assert holdings for non-collateral token definitions (stablecoin holdings).

File summaries

File	Description
programs/integration_tests/tests/stablecoin.rs	Adds the full-lifecycle stablecoin zkVM integration test and a generalized fungible-balance assertion helper.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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