feat(stablecoin): add collateralization-check helper

[gravityblast]

Adds stablecoin_program::checks::assert_position_is_collateralized, the shared
spec §6.2 invariant used by withdraw_collateral (post-decrement) and
generate_debt (post-mint).

Pure function over scalars — the callers project the accumulator and redemption
price forward to now (§5.3) and pass them in, so the projection isn't computed
twice per instruction. Uses the cross-multiplied form the spec prescribes, in
U256, so no intermediate rounding enters the comparison. Zero-debt positions
short-circuit to pass.

8 unit tests: zero debt with and without collateral, comfortable surplus, the
exact boundary and one unit below it, exactly 1.5x and one unit below it, and
accumulator growth flipping a passing position to failing.

First of eight issues in Plan 3 (#173). No Instruction variant and no guest
change, so the IDL is untouched.

closes #174

[Copilot]

Pull request overview

Adds a reusable, spec-aligned collateralization invariant check to stablecoin_program so upcoming position-lifecycle instructions can share one canonical implementation and test suite.

Changes:

• Introduces stablecoin_program::checks::assert_position_is_collateralized, implementing the §6.2 invariant using U256 intermediates and a zero-debt fast-path.
• Adds focused unit tests covering boundary, near-boundary, and accumulator-growth scenarios.
• Wires the new module into the crate exports and adds the required direct dependency.

Reviewed changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
programs/stablecoin/src/lib.rs	Exposes the new checks module from stablecoin_program.
programs/stablecoin/src/checks.rs	Implements assert_position_is_collateralized and adds unit tests for key invariant edges.
programs/stablecoin/Cargo.toml	Adds alloy-primitives as a direct dependency for U256 use in stablecoin_program.
Cargo.lock	Records the new stablecoin_program -> alloy-primitives dependency in the workspace lockfile.
programs/stablecoin/methods/guest/Cargo.lock	Updates the guest lockfile to reflect the new dependency in guest builds.

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