Fix lambda parameter scope restoration to prevent memory leak (#1560)

[arnaudgelas]

This commit implements proper scope restoration for lambda parameters, addressing FIXME #1560 and preventing scope pollution in nested lambdas.

Problem

The previous implementation did not restore lambda parameter registers after executing the lambda body. This caused:

1. Parameter scope pollution: Nested lambda calls would overwrite parent scope parameter values without restoration
2. Memory leak: Previous parameter values were not restored, accumulating state
3. Incorrect semantics: Lambda parameter bindings should be local to the lambda

Example issue:

let f = \ x => x + 1
let g = \ x => f(x)
g(5)  // This would corrupt x in g's scope

Solution

The fix implements a save-restore pattern for lambda parameter registers:

1. BEFORE executing lambda: Save the current value of each parameter register
2. DURING execution: Overwrite registers with new parameter values
3. AFTER execution: Restore each register to its previous value

This ensures lambda parameters are truly local and don't pollute outer scopes.

Implementation Details

• Save previous values before parameter binding (lines 482-487)
• Execute body with new parameter values (line 489)
• Restore previous values after execution completes (lines 491-496)
• Error handling: Restoration happens whether execution succeeds or fails

The fix maintains the Either monad pattern by restoring values even if the body execution returns an error.

TypeScript Equivalence

The TypeScript evaluator does not have this issue due to its function closure model. This Rust fix brings semantic parity with the intended behavior: lambda parameters should be locally scoped and not affect outer bindings.

Testing

This change affects only the Rust evaluator's lambda execution path. Existing tests in simulator_tests.rs should still pass as they don't exercise nested lambda parameter shadowing at the register level.

Fixes: #1560
Related: fix-semantic-divergence branch

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