fix: replace panicking .expect() / assert! on PC access counter overflow in ProgramMemCheckChip

[amathxbt]

Problem

ProgramMemCheckChip::fill_main_trace panics in two places when a PC is accessed more than u32::MAX times:

let new_access_counter = last_access_counter
    .checked_add(1)
    .expect("access counter overflow"); // panic 1
// ...
assert!(!carry_bits[WORD_SIZE - 1]); // panic 2 — redundant overflow check

An adversarially crafted program with a tight infinite loop (j 0) will eventually push the counter past u32::MAX, triggering an unconditional panic that kills the prover process. This is a denial-of-service vector: a malicious guest program can crash the prover without producing a verifiable trace.

Fix

• Replace .expect() with a let…else that returns early on overflow. The constraint system's carry-overflow constraint (Don't allow overflow in add_constraints) will reject the resulting trace as unsound — no panic required.
• Downgrade the two redundant assert! calls to debug_assert! since checked_add already guarantees the invariant on the non-overflow path.

[github-actions]

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[amathxbt]

I have read the CLA Document and I hereby sign the CLA