One disclosure draws every expandable section

[maximsan]

Why

Closes #596. Five hand-rolled expanders, four timings and two marks between
them, become one Disclosure.

Watch for

• Every module on Path now opens and shuts. disclosure.jsx ties
  collapsible={false} to panel open, so the module you are working through
  was pinned open with no caret. The owner ruled that behaviour wrong: a caret
  on every module the learner can reach, open and collapse at will. The active
  one still opens itself on arrival. A module you have not reached keeps
  neither caret nor lessons — there is nothing behind one to open.
• Disclosure states isOpen rather than inferring it from collapsible,
  which is where it departs from disclosure.jsx. That inference is what broke
  Path, and it misfires at the design's own Reference site too: it passes
  shut-while-locked and the component overrules it.
• A shut panel is not mounted. Its children are dropped once the collapse
  finishes, so PathChallengeNode and the guide rows stop watching their
  providers while shut. Same end state as the old if (open); what moved is
  when the drop happens — after the animation rather than at the tap.
• For later is shut on arrival, as the design has it; the parked brews were
  always visible before. Its count rides in the header so a shut list still
  says how much is behind it.
• Reference's header now matches a module's — a 32-px glyph column, caption
  at the title's left edge. Its lock stays 16 where a module's is 13.
• The header row centres its glyph. CrossAxisAlignment.baseline — the
  design's alignItems: 'baseline' — puts a child reporting no baseline at the
  top of the row, and every mark reports none. Proper baseline alignment lands
  within half a pixel of centre for a 16-px mark beside a 29-px title, so the
  row centres and a test holds it there.

Not here

• Reference's locked shelf stays shut. Opening it, as the design's inference
  would, adds "{n} more unlock as you learn" under a caption that already
  says it — and that caption is
  ADR-0016's,
  so it wants a ruling rather than a side effect.
• Coffee Duel's review disclosure: v1 does not ship the Duel (One disclosure draws every expandable section — practice groups, Path's modules, Reference, For later and the FAQ (11 Sep drop) #596, Out of
  scope).
• Practice sub-groups: The practice list groups lessons by module and games by kind, collapsibly (10 Sep drop) #582, which this unblocks. Every slot it names — header
  padding, min height, glyph size, trailing gap, panel padding — is on the
  widget.

Checks

Driven on a simulator and reviewed as pictures — Path, Today's practice list
and the FAQ — then thrown away.