fix(verifier): crash and mis-parse fixes in math verification engine

[CodeWhitaker]

7 findings in services/verifier/ (Python/SymPy), each reproduced before fixing. The Next.js side (guards, quota reserve/refund, Lemon Squeezy HMAC) was reviewed and found solid.

Fixes

1. [High] Tuple-shaped parse results (final_answer="2, 3") crashed verify_payload -> HTTP 500. parse_math_expression now returns None for non-Expr.
2. [High] SymPy AttributeError inside simplify().equals() on \int_0^\infty e^{-x} dx -> HTTP 500. try/except in expressions_match; numeric fallback still applies.
3. [Med] \sinh x was mangled to sin(h)*x. Hyperbolic functions handled + (?!h\b) guard.
4. [Med] Lazy regex captured just \ for \to\infty/\infty bounds. Braced-group-or-bare-token bound patterns + \infty->oo mapping.
5. [Med] Limits in variables other than x/y/z/t were silently computed in x. Symbol(var_name) fallback.
6. [Med] Linear-system matcher false-verified "the max = 5, y = 1" (matched x= inside "max"). Leading \b added.
7. [Low] Bare e parsed as a free symbol instead of Euler's number. "e" -> E in LOCAL_SYMBOLS.

Verification

• 13/13 -> 23/23 verifier tests (10 new regression tests, one per fix)
• Zero TS files touched; TS suites covered by CI

Deploy note

Redeploy the verifier service after merging. No secrets or infra changes needed.

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