Add article-gap features + 5-stage walkthrough

[gititya]

Summary

• Implement all three FUTURE-FEATURES.md features: AI supervision load (A), assisted work with AHT multiplier (C), volume growth reframe (B)
• All features ship OFF by default — defaults reconcile to naive 31 / model 44 / $4.46 / ~$39.4k/wk
• Refactor single-screen dashboard into a 5-stage sequential walkthrough (Staffing → Oversight → Assisted work → Growth → Summary) with stepper navigation, walkthrough text, summary chips, and stage-specific insight callouts
• Model code (derive, sweep, readInputs, classifyRegime) untouched — stage management is pure visibility/CSS

Test plan

• Open index.html — Stage 1 shows 44 humans / $4.46 / $39.4k at defaults
• Click through all 5 stages — correct inputs appear, previous collapse to chips
• Navigate backward via stepper/chips — later stages re-hide
• Stage 2: set supMin=4 → headcount rises, floor rises
• Stage 3: set assistShare=50%, factor=0.60 → headcount drops
• Stage 4: set growth=30%, switch to Growth → reframe message appears
• Stage 5: all inputs shown read-only with final KPIs
• Edge cases: 0% and 100% coverage — no NaN/Infinity

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