Extend LiftImperativeExpression pass to handle assert/assume

[keyboardDrummer-bot]

Extends the LiftImperativeExpression pass to handle assert and assume nodes, as described in #43.

Changes

Strata/Languages/Laurel/LiftImperativeExpressions.lean:

• Added .Assert and .Assume cases to transformExpr — when these appear in expression context (e.g. inside block expressions), their conditions are transformed with a fresh substitution context (to lift any assignments within), and the assert/assume itself is lifted to prepends using the correct ordering pattern.
• Updated transformStmt for .Assert and .Assume to always transform their conditions, lifting assignments and imperative calls (previously only handled nondet holes).
• Added .Assert and .Assume to containsAssignmentOrImperativeCall for recursive detection of impure expressions within assert/assume conditions.

StrataTest/Languages/Laurel/LiftExpressionAssignmentsTest.lean:

• Added test for the example from the issue: var y := { assert x > 0; 3 } and assert (x := 2) == 2
• Added test for assume (x := 2) == 2

StrataTest/Languages/Laurel/Examples/Fundamentals/T2_ImpureExpressionsError.lean:

• Removed error expectations for assert with assignments/imperative calls in procedure bodies, since these are now handled by the lift pass (errors remain for contracts/functions which are pure contexts).

Prepend ordering

For lifted nodes, the prepend ordering follows the pattern from the issue:

argPrepends ++ liftedNode ++ prePrepends

ensuring child prepends come before the lifted node itself, and outer prepends come after.

Fixes #43"

[keyboardDrummer-bot]

The "Run internal benchmarks of Strata" failure is not caused by the code changes in this PR. It's a systemic issue affecting all PRs on this fork — the CI workflow's benchmark job is hardcoded to pull source from strata-org/Strata.git but the commit SHA only exists in keyboardDrummer/Strata, so CodeBuild can't find the commit and fails immediately (~1 minute runtime). The same failure occurs on PR #44 and other PRs from this fork.

Locally, the build and all 591 test jobs pass successfully with no errors.