fix: honour code-window-collapse false on a block

[mcanouil]

code-window-collapse="false" could not turn collapsing off for a block.

A document that sets collapse folded every block, and a block that opted out was folded with the rest.
The call site read the block override with a bare or, so an explicit false was indistinguishable from an absent attribute and fell through to the document setting.
The comment above the reader already said this must not happen.

The fix has two halves, because the first alone changed nothing a reader would see:

• read_block_collapse answers whether the block decided, so an explicit false disables collapsing while an absent attribute still inherits.
  A value the schema does not accept keeps the old behaviour, inheriting while the check reports it.
• A block that turns collapsing off carries cw-collapse-none, and the injected script honours it as a hard off.
  Without that marker the script reads a missing class as "this block said nothing" and applies the document default anyway.

A fixture covers both blocks, one opting out and one inheriting, and asserts that the two differ and that the document setting still reaches the page.
The suite is 21 passed, 0 failed.

Two further findings from the review are not addressed here:

• A block with code-window-enabled="false" and a filename still gets a .code-with-filename wrapper from Quarto, so with a collapsing document the script styles and folds a block that opted out of the extension entirely.
  I reproduced it and left it for its own change, since it needs a fixture of its own.
• checker:attributes runs for every format in the disabled branch while checker:options is gated on the acting formats, so a docx or latex render reports per-block findings only when the filter is off.

Verified locally: tests/run.sh 21 passed 0 failed, full quarto render docs (exit 0, no warnings), markdownlint-cli2 with my config (0 issues).