Publish pair-frequency model validation

[jamiepratt]

Summary

• publish the continuous pair-frequency logistic v2 checkpoint and its negative promotion result
• add versioned fixtures, tuning/held-out/stress outputs, deterministic verification, and CLJ/CLJS parity tests
• add optional equation and terminology explanations across the vocabulary series
• fix light/dark contrast for explanatory text, controls, and interactive states

Tests

• clj-paren-repair on all changed Clojure and ClojureScript files
• clojure -M:test-clj — 14 tests, 58 assertions
• clojure -M:test-cljs — 10 tests, 45 assertions
• clojure -M dev/verify_pair_frequency_logistic_v2.clj — doubled-grid convergence passed
• rendered all three affected articles with Clay and Quarto
• internal-browser verification: simulators, labels, console, light/dark contrast, and 390×844 mobile layout
• quarto render site — all 229 pages completed