docs: Fugue Explorables — interactive, data-first documentation rehaul (#39)

* docs: Fugue Explorables — interactive data-first docs rehaul

Adds an Explorables section: six interactive pages in the tradition of
Bret Victor's explorable explanations and 3Blue1Brown's visual math,
built on a shared dependency-free canvas library (docs/fugue-viz.js,
~5k lines total, no CDNs, no build step, theme-aware, reduced-motion
safe, seeded/reproducible).

Every widget centers on live data: draggable observations with priors
and posteriors updating in real time. Metropolis and HMC share one
Bayesian linear regression as twin-panel views (data space with
posterior spaghetti <-> parameter space with the sampler), with live
split-R-hat/ESS implementing the 0.2.0 diagnostics math in JS. The
monad page performs a Normal-mean model one effect at a time over a
draggable dataset with its exact conjugate posterior. SMC shows the
filtering distribution and mean band over its particle cloud.

Also a 0.2.0 truth pass over all existing pages: fixed broken prob!
snippets (raw for-loops the macro cannot parse), a duplicate-address
hierarchical sample that would panic, a fabricated ScoreGivenTrace
API, dead example references, wrong repo URLs, and stale memory-
subsystem claims; every remaining snippet verified against src/.

Fixes a live site bug: default-theme was "dark", which is not an
mdbook theme name, so light-OS visitors got the light CSS fallback
while the html class claimed dark; now navy, and the viz library
detects dark/light from mdbook's --bg token instead of class names.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DEAKcbnLVP8iSXub2Pqor2

* docs: alive everywhere — autoplay, ambient micro-widgets across all pages

Every widget now autoplays when scrolled into view (pausing offscreen;
reduced-motion gets a rich pre-rendered frame, never an empty axis) and
heroes pre-warm so first paint already shows a running system.

Adds docs/viz/inline.js: ten parameterized ambient micro-widgets
(dist-strip, posterior-morph, trace-ticker, rhat-spark, shrinkage,
regression-mini, mixture-resp, logistic-boundary, elbo-climb, abc-eps)
with real math inside (split-R-hat and ESS ported from
src/inference/diagnostics.rs, EM responsibilities, conjugate updates),
embedded at the point of discussion across ~20 pages — including a
deliberately unconverged rhat-spark on debugging-models and a live
partial-pooling shrinkage widget on hierarchical-models.

Removes all "new in 0.2.0"-style callouts (13 sites incl. the
distributions widget's selector group) — version history lives in the
CHANGELOG, not the docs.

Browser-verified: all 25 widget-bearing pages animate on arrival, zero
console errors, zero >50ms long tasks during ambient play on the
busiest pages, reduced-motion renders rich static frames (fixed a
resize race that blanked paused micro-widgets).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DEAKcbnLVP8iSXub2Pqor2

* docs: touch-grade smoothness pass + in-page support figures

Fixes the root cause of mobile bugginess: .fv-canvas set touch-action:
none on EVERY canvas, so thumb swipes over any figure ate page scroll.
Canvases now default to pan-y; full gesture capture is opt-in via a
shared FV.drag manager (pointer capture, claims the gesture only when
the touch actually hits a draggable, >=22px hit inflation on coarse
pointers, grab halo). Killed the same class in inline.js (unconditional
preventDefault on touchmove) and in every hero's hand-rolled drag code;
monad's machinery strip no longer steals scroll swipes as Steps;
flash decay is time-based; layouts audited at 390px and 320px.

Adds docs/viz/minis.js — eight page-support figures woven into the
explorables' explanation sections: autocorrelation decay and a live
sigma-sweep on Metropolis, the double-well momentum-vs-random-walk
strip and a divergence-cliff scatter on HMC, an animated bind-chain
on the monad page, sequential-update small multiples on Anatomy, an
ESS timeline on SMC, and a natural-return-types flow on Distributions.

Mobile-verified via emulated 390x844 touch contexts with CDP touch
events: swipe-over-widget scrolls the page, point-drags move points
without scrolling, zero console errors, zero >50ms long tasks; full
desktop regression green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DEAKcbnLVP8iSXub2Pqor2

* docs: rhat-spark rewrite — real split-R̂ on trailing window, chains visibly converge

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018yo1jdkckG7yQ9eBqLbaDB

* docs: rhat-spark — make the collapse visually read (envelope must shrink)

Starts at ±3.4 stationary SDs with thin warmup threads (SD 0.32), a held
separated phase, then a smoothstep funnel onto the shared band. Before,
starts ±2.2 with SD 0.65 meant the merged band spanned nearly the same
pixels as the separated chains — nothing visibly converged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018yo1jdkckG7yQ9eBqLbaDB

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: alexnodeland

docs/book.toml

@@ -10,11 +10,15 @@ text-direction = "ltr"
 create-missing = true
 
 [output.html]
-default-theme = "dark"
+# "navy" is mdbook's dark theme; the previous value "dark" is not a real
+# mdbook theme name, so light-OS visitors silently got the light CSS fallback
+# while the html class still claimed dark.
+default-theme = "navy"
+preferred-dark-theme = "navy"
 git-repository-url = "https://github.com/alexnodeland/fugue"
 site-url = "/fugue/"
-additional-css = ["./mdbook-admonish.css"]
-additional-js = ["mermaid.min.js", "mermaid-init.js"]
+additional-css = ["./mdbook-admonish.css", "fugue-viz.css"]
+additional-js = ["mermaid.min.js", "mermaid-init.js", "fugue-viz.js", "viz/anatomy.js", "viz/monad.js", "viz/metropolis.js", "viz/hmc.js", "viz/smc.js", "viz/distributions.js", "viz/inline.js", "viz/minis.js"]
 
 [output.html.fold]
 enable = true