Add quantified economic adoption thresholds from fork simulation study

[pfoytik]

Related to #83

Adds a short paragraph and footnote to "Impact of Economic Node adoption rates on chain split risks," putting explicit numbers on the section's existing low/medium/high adoption framing:

• E < 0.50: cascade floor, new rules can't win regardless of miner commitment
• E ∈ [0.50, 0.82]: inversion zone, miner commitment becomes the deciding variable
• E > 0.82: economic override, new rules win regardless of miner commitment
• E ≈ 0.74: self-sustaining point within the inversion zone, where the price signal stops depending on continued miner cooperation

These come from a 2026 simulation study of a hypothetical contested Bitcoin soft fork (arXiv:2608.05461, cited in the new footnote), not from Bitcoin's actual history. The footnote says so explicitly, this is offered as a mental model for exploring the dynamic BCAP already describes qualitatively, not a claim about validated real-world thresholds.

This is item 1 of the roadmap from #83, a self-contained two-line addition to existing prose, not a restructure. No other files need changes (index.html fetches README.md live, and bcap_v1.0.pdf is a separately-built artifact outside this PR's scope).

Open to adjusting wording, threshold representation, or hedging language based on feedback.