feat: bring predicted extinction date from year 5174 to ~2030

[Copilot]

The "Days Until Extinction" life-blocks countdown targeted MILESTONES[MILESTONES.length - 1] (biosphere_collapse at 10 quintillion tokens), projecting to year 5174 — not viscerally meaningful. This decouples the countdown horizon from array position via a new extinctionMarker boolean field.

Changes

• milestones.yaml / milestones-data.js — adds human_extinction at 77 quadrillion tokens (~Feb 2030) with extinctionMarker: true; all 33 existing milestones preserved
• script.js — LB_LAST_MILESTONE resolves via MILESTONES.find(m => m.extinctionMarker) with fallback to last milestone
• scripts/build-milestones.js — emits optional extinctionMarker field; adds validation that at most one milestone carries the flag
• tests/death-clock.test.js — asserts extinctionMarker: true appears on ≤ 1 milestone

[github-actions]

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