Improve predicate observation handling for convergence checks

[Copilot]

Predicate-based conditions were not marking their target variables as observed, leaving convergence checks to fragile codegen heuristics.

• Observer analysis: Track the last assigned variable per scope, reuse a shared predicate set, and recursively detect predicates to mark the correct variable as observed.
• Cleanup: Remove redundant identifier predicate branch for clarity.
• Tests: Add compiler test ensuring loop while not converged produces a call to eigen_check_converged.

Example:

x is 0
loop while not converged:
    x is x + 1  # now marked observed; codegen emits eigen_check_converged

Original prompt

Look for areas of improvement

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