PR1: calc.py hardening + scripts/ relocation

[artvandelay]

Closes the high-severity bugs from stress-test rounds 1 + 2:

• Strict input validation (rejects null, NaN, non-numeric, negative, zero where invalid) with clean exit 2.
• Honest infeasible output: savings_pct=0, weekly_savings_usd=0 when VRAM insufficient.
• New top-level warnings[] array (e.g., experiments_multiplier clamp).
• MoE sanity check (active_params_b <= total_params_b).
• traffic_pattern enum validation; cold_per_query requires hot_hours_per_week.
• File moved from calc.py to scripts/calc.py (Anthropic convention); SKILL.md and README path strings updated; tests + CI updated.

All existing tests pass plus new tests in TestHardenedValidation, TestInfeasibleOutput, TestWarnings.