Implement proofs for ContinuousWithinAt.tfae and Exercises 9.4.6 & 9.4.7 #438
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This PR implements missing proofs for Section 9.4. Specifically, I established the equivalence of continuity definitions in "ContinuousWithinAt.tfae" (Prop 9.4.7) by bridging the topological, sequential, and metric definitions via manual logical rewriting. I implemented the proof for "ContinuousOn.restrict" (Exercise 9.4.7), demonstrating that a function continuous on a set X remains continuous on any subset Y of X. Finally, I completed Exercise 9.4.6 by proving the continuity of polynomial functions "Continuous.polynomial" using induction and the course's sum/product rules.