docs: correct the two places the user-facing docs are wrong (#206, #207)

[BernardJen]

Closes #206 and #207.

#206 — wrong, not merely stale

docs/USER_GUIDE.md described the screensaver as "a bouncing logo". There are 30.
README.md's feature list carried the same line. The guide's shortcut table also omitted
V, + and - entirely, so an operator reading it could not discover the screensaver
controls at all — which is exactly how the omission surfaced: the owner had to ask for
those keys rather than read them.

• README feature line rewritten.
• User guide's Screensaver section rewritten, including that each saver looks different
  every activation because its choices are clock-seeded.
• V, + and - added to the shortcut table.
• All 30 listed in rotation order, which is the order + and - step through.
• The split-flap board's status explained: it is the no-signal display, appears
  immediately rather than after five minutes, and cannot be reached by stepping. That
  distinction was documented nowhere a user would look.
• Weather and Art-Net covered at an operator's level, both flagged off by default, detail
  left in README rather than duplicated.

The list came from registry.js, not from the issue — the issue says explicitly to
check against code because the issue will itself age. Cross-checked against
structure-baselines.js, the other generated list, as an independent count.

#207 — two workflows documented nowhere

.claude/CLAUDE.md's command block omitted npm run bench and npm run baselines, so the
only way to find them was to read package.json.

• Both added, with the baselines caveat stated where someone will hit it: it rewrites all
  30 entries, so a single-saver redesign should edit one line by hand.
• The SwiftShader-versus-real-GPU contrast between shadercheck and bench spelled out —
  including that getting it backwards is what led several PRs to claim frame cost could not be
  measured headlessly when it could.
• Preview harness keys documented in prose. Verified against the key handler rather than
  copied from the issue, which turned up four the issue omitted — R, the arrow keys, H
  and F — and one genuine collision worth a warning: +/- step savers in the shipped app
  but adjust bloom in the harness.

The documented selector example did not work

CLAUDE.md's one worked example of the preview flag was white-particles, and the matcher
stripped whitespace but not hyphens, so it silently fell through to the first saver. The
worst kind of doc error — it teaches the reader the feature is broken.

Fixed in the matcher, not the prose, because white-particles is what a reader will type
and it matches the file on disk. All four forms now resolve: white-particles,
whiteparticles, White Particles, white_particles.

Verified end to end rather than from the regex: navigating to #white-particles logs
[Screensaver] Started: White Particles with no fallback warning.

A test, because the list has already gone stale twice

#206 suggests generating the list from registry.js. test/docs-screensaver-list.test.js is
the cheaper half — it needs nobody to remember to run a script, and it fails in CI the moment
prose and code disagree. It resolves the registry order from source and checks:

• the documented list matches the registry exactly, in order (order matters, since the
  guide claims it is the +/- order);
• the stated count is right in both files;
• neither file says "bouncing logo";
• the V and +/- rows are present;
• the split-flap board is not in the rotation.

Fault-injected both ways — dropping a saver from the list, and reintroducing the
bouncing-logo line — and each fails it.

Verification

• Branched from main (3201dbd).
• npm run lint clean.
• npm test — 412 passed (407 + 5 new).
• Selector fix confirmed live in the preview harness.

Not verified / left open

• docs: the user-facing documentation is 19 releases behind and wrong in two places #204 stays open. Its changelog half was docs: bring the changelog up to date, 2.2.2 through 2.13.0 (#204) #205, its two wrong-documentation halves are
  this PR, and its last part — chore: TODO.md is stale and superseded by issues — delete it or mark it historical #208, TODO.md being stale — is untouched.
• The saver descriptions are not written. The guide lists all 30 by name in rotation order,
  which is what the issue asked for; one-line descriptions of each would be a larger writing
  job and nobody has asked for them.
• The generator script docs: the user guide still calls 30 screensavers "a bouncing logo", and hides V / + / - #206 floats as an alternative is not built. The test makes the list
  self-checking, which addresses the staleness risk without adding a build step, but it does
  mean the list is still updated by hand.