Keep a valid selection when the editor value is replaced

[bmesuere]

TODO (author) — drafted by Claude: proofread, verify and remove this line.

This pull request passes an explicit cursor when CodeMirrorEditor replaces its whole document, so an edit made after an external value update no longer throws RangeError: Invalid change range 482 to 0 (in doc of length 482).

Sentry: DODONA-FRONTEND-2CM, thrown from @codemirror/state after clicks in p-code-runner.

Cause. dispatchChange dispatches {from: 0, to: doc.length, insert} without a selection, so CodeMirror maps the current selection through the change. SelectionRange.map maps a non-empty range's from with assoc 1 and its to with assoc -1; for a range strictly inside the replaced text that gives from = new length and to = 0. The next paste, delete or typed character at that selection builds a change from the inverted range and throws. An empty cursor is not affected, which is why this only shows up with text selected. @codemirror/state 6.7.6 (latest) maps ranges the same way.

dispatchChange runs for every value that does not come from the editor itself: code set by the host page, and effectiveCode changing when test code is added or removed. Typing does not reach it, since the setter returns early when the value equals the document.

• The dispatch now sets the cursor to the old selection head, clamped to the new length. That keeps the cursor where it was when the host sends back nearly the same text, and matches CodeMirror's own behaviour for an empty cursor otherwise.
• The fix is in CodeMirrorEditor, so the code editor, the batch input editor and the file editor (CodeEditor, BatchInputEditor, FileEditor) all get it.

Manual testing instructions

• In the scratchpad, select a few characters in the middle of the code, then make the code change from outside the editor (for example by adding or removing test code, or setting papyros.runner.code from the console). Type or paste: the text is inserted at the cursor and nothing is logged to the console.

Checklist

• Tests: one case in CodeEditor.test.ts that selects text inside the document, replaces the value and inserts at the selection. It throws the Sentry error before this change.
• AI: Claude Code investigated this during Dodona's weekly Sentry triage and wrote the fix.

Reaches production once Dodona bumps @dodona/papyros.

[bmesuere]

🤖 Reviewed. This fixes the same concrete CodeMirror selection corruption seen in Dodona, at the shared Papyros editor layer. Preserving the old head, clamped to the new document, is a sensible host-update behavior and prevents the next edit from using an inverted range. Focused tests, lint, and typecheck pass locally. No blocking findings.