JavaScript: Ensure workspace is collapsed when solution is revealed

[kylemonette]

This PR deals with two workspace/solution-pagination fixes, continuing off the changes made in PR #3114 .

1. Workspace didn't collapse when a solution was revealed
   Workspace was always stretched to fill unused space on the page, even after the question's hint/answer/solution had been revealed. This left a large gap between a question and its "Solution" block.

2. Embedded videos reloaded unnecessarily during repagination
   Toggling solution visibility triggers a settle loop that can move page rows (and therefore any <iframe> inside them) between .onepage containers several times before layout stabilizes. Each move caused the iframe to reload, so a single checkbox toggle could reload a video multiple times.

New functions:

• anySolutionTypeVisible() — Returns whether any hint/answer/solution is currently visible on the page. Checks effective visibility (the element itself and every ancestor free of the hidden class), not just :not(.hidden) on the element alone. This distinction matters because a live PreTeXt knowl nests its content in a .knowl__content div that never carries hidden directly — only its .born-hidden-knowl ancestor wrapper does. A naive :not(.hidden) check matches that inner div regardless of the collapsed ancestor, which caused every workspace on a page to incorrectly read as "a solution is visible" from the moment the page loaded, before the reader had revealed anything.

• withIframesDetached(fn) — Wraps a batch of repagination work. Detaches every <iframe> in the printout (swapping each for a same-sized placeholder div, so height measurements stay accurate) before fn runs, then reattaches them once fn resolves. This caps a solution-visibility toggle at reloading an embedded video once, instead of once per intermediate row move during the settle loop.

Modified functions:

• setInitialWorkspaceHeights() — Now collapses workspace to 0px instead of its authored height whenever anySolutionTypeVisible() is true, freeing that space for pagination once it's no longer needed as blank writing room.

• adjustWorkspaceToFitPage() — Skips the pass that stretches workspace boxes to fill unused page space whenever anySolutionTypeVisible() is true, leaving workspace at its (now-collapsed) authored height instead.

• collapseSpilloverPages() — Once a solution is visible, workspace is pinned at its collapsed floor and will never shrink further, so the normal "merge first, let the caller re-split if it doesn't fit" order no longer converges — it just oscillates every settle tick. This function now checks fit before merging in that case (via a new workspaceIsFixed flag), and puts a spillover page's content back rather than flip-flopping if it doesn't actually fit.

• applySolutionVisibility() — The whole toggle body now runs inside withIframesDetached(), since the repagination/collapse work it triggers can move a row several times over the course of one toggle.

[kylemonette]

Follow-up: Revealing a solution can also free up space, not just use it

The previous fix made workspace collapse once a solution is shown. But applySolutionVisibility() still assumed that showing a solution could only ever need more room on the page, so it only tried to reclaim space when hiding solutions, never when showing them.

Showing a solution can now shrink a page too, because the workspace collapses at the same time. So a page that had already spilled its content onto a new page can end up with room to fit that content back.

Both showing and hiding now try to pull spilled-over content back onto the previous page first, and only push content to a new page if it still doesn't fit. This is safe in both directions because pulling content back only happens if it actually fits (from the previous fix).

[oscarlevin]

Some of the work here might be subsumed by that of #3154, which should be merged first I think.