FEAT: Collapse solutions by default under solution_follow_exercise - #86

The `gateddirective` test root sets `html_theme = "sphinx_book_theme"`, which
is currently broken on Sphinx 6 and 7 in CI: a newer pydata-sphinx-theme calls
`_get_toctree_ancestors`, which does not exist in those Sphinx versions, so
every test using that root fails with a ThemeError. Five pre-existing
`test_gateddirective.py` tests fail the same way; that breakage is unrelated to
this branch and is left alone here.

Adds a self-contained gated fixture to the `mybook` test root, which uses
alabaster and is unaffected, and points the gated collapsed test at it. The new
fixture is also much lighter than the `gateddirective` one, which executes
matplotlib code cells.

Verified the fixture genuinely exercises the merge path rather than falling
back: the rendered solution carries `class="solution dropdown admonition"`, the
title resolves to "Solution to Exercise 7 (A gated example)", the intervening
content is merged into the admonition, and no solution-end marker survives.

Also adds a default-off counterpart so the gated path is covered in both
directions.

Refs #85

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Three fixes from an adversarial review of the initial implementation.

1. Drop `sphinx_design` from `TOGGLE_EXTENSIONS`. It does not provide the
   `dropdown` class: its dropdown is a directive emitting `.sd-dropdown`, and
   its stylesheet ships no rule for a bare `dropdown` class (verified — the
   compiled CSS contains only `.sd-dropdown`). Listing it suppressed the
   warning in exactly the case the warning exists for, and because Jupyter Book
   loads sphinx-design by default, a Jupyter Book project without
   sphinx-togglebutton would have silently rendered every solution expanded
   with no diagnostic.

2. Give the warning `type="exercise"` / `subtype="solution_collapsed"`. It was
   untyped, so it could not be suppressed and made `-W` builds fail outright
   for projects that supply their own `.admonition.dropdown` CSS instead of
   loading the extension — a setup the check's own docstring anticipates.
   `suppress_warnings = ["exercise.solution_collapsed"]` now silences it while
   still applying the class.

3. Document that collapsing hides content by zeroing its height rather than
   removing it, so outputs that measure themselves at load time (plotly, bokeh,
   ipywidgets, altair) render at zero size inside a collapsed solution. This
   matters for exactly the executable-book audience the option targets. Static
   images including matplotlib are unaffected; `:class: toggle-shown` is the
   per-directive escape hatch.

Also hardens the test fixtures: the two new toctree entries move to the end of
`test-mybook/index.rst` and the gated fixture's exercise becomes `:nonumber:`,
so neither can shift the global exercise numbers baked into ~40 committed
regression fixtures. A comment records the constraint for future entries.

Adds a test that the warning is suppressible while the class is still applied.

Verified: 127 tests pass; the new tests pass on Sphinx 6.2.1, 7.4.7 and 8.2.3;
docs build adds no new warnings.

Refs #85

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

These six .sphinx9 baselines were generated accidentally while reproducing an
unrelated CI failure in a scratch environment that had been upgraded past the
supported Sphinx range. pytest-regressions writes a new baseline when it finds
no file for the running version's suffix, so they were created as a side effect
rather than deliberately.

They do not belong in the tree: the project supports sphinx>=6.1,<9, so there
is no CI job that would ever read them, and they record output produced with a
theme/Sphinx combination the suite does not target.

Refs #85

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Folds collapsing into what the `solution_follow_exercise` exercise style
means, rather than leaving it as an unrelated switch authors must find.

That style places each solution directly beneath its exercise, which is exactly
the layout the reader feedback in #84 was about: an adjacent, fully visible
solution is too tempting to look at. Treating the fold as part of the style
means the option authors already reach for does the right thing by default.

`solution_collapsed` becomes tri-state to make this expressible:

  unset (None)  follow exercise_style - collapsed under solution_follow_exercise
  True          always collapse
  False         never collapse

The `None` default is load-bearing, not stylistic. Sphinx cannot distinguish
"unset" from "explicitly set to False" through the public config API, so with
the previous `False` default there would have been no way to switch the style's
implied collapsing back off.

Resolution lives in `utils.solutions_are_collapsed(config)` rather than being
written back into the config at `config-inited`. That keeps the raw tri-state
readable, so the warning can tell whether collapsing was asked for or implied,
avoids mutating config other extensions may read, and needs no rebuild-trigger
handling of its own since `exercise_style` is already registered "env".

The missing-togglebutton warning now adapts. An author who opted in explicitly
is told to add the extension; an author who only set `exercise_style` and never
asked for collapsing is told how to switch it off as well:

  exercise_style='solution_follow_exercise' collapses solutions by default, but
  'sphinx_togglebutton' is not loaded, so they will render expanded. Add
  'sphinx_togglebutton' to your extensions, or set solution_collapsed = False
  to keep solutions expanded.

This is a behaviour change for projects on `solution_follow_exercise`, so it is
documented as one: a "Changed" entry in the CHANGELOG leading with how to
restore the old rendering, and new v1.3.0 release notes opening with the change
and the one-line opt-out. The syntax guide gains a value table for the
tri-state, and the Solution Title Styling section now lists collapsing among
what the style does. Jupyter Book ships sphinx_togglebutton in its default
extension list, so Jupyter Book projects need no change.

Six new tests cover the interaction: the style collapsing by default, explicit
False overriding it, explicit True agreeing with it without duplicating the
class, both warning texts, and silence when the implied collapse is switched
off.

Verified: 133 tests pass; the collapsed and exercise_style suites pass together
on Sphinx 6.2.1, 7.4.7 and 8.2.3; docs build adds no new warnings.

Refs #84, #85

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

The gated collapsed tests use the RST `.. solution-start::` form in the
`mybook` test root. This adds the MyST ```{solution-start}``` counterpart in
the `gateddirective` root, so the directive-option parsing both parsers feed
into is exercised from each side.

That root was unusable while its `sphinx_book_theme` setting was breaking CI on
Sphinx 6 and 7, which is why the earlier gated test was written against
`mybook` instead. #88 has since switched it to alabaster, so it is available
again.

The `mybook` fixture stays as the primary gated test: it is self-contained, it
sits alongside the rest of the collapsed suite so confoverrides stay
consistent, and it avoids executing the matplotlib code cells the
`gateddirective` root builds.

Verified: 134 tests pass.

Refs #85

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>