fix: swallow a following decorator when a removal window reaches into it

[LaatonWalaBhoot]

Closes #82

Two changes, both in the multi-paragraph removal paths:

• isLastDecoratorPartiallySelected is now computed in document coordinates (window end strictly inside the last item's decorator span, and only for list items). The previous form compared the decorator piece's buffer offset against document offsets, so the partial-decorator swallow stopped firing once decorators sat deep in the ADDED buffer.
• TextUpdateTransaction gets the replace-side mirror of extendPastOrphanedDecorator (Deleting across a paragraph boundary up to a list item's decorator orphans the decorator in the live text #67/TextDeletedTransaction): a removal window that consumes the terminating line break and stops at the next item's start extends one char into the decorator and re-dispatches, so the existing handling swallows it whole and renumbers. One difference from the delete rule: a non-empty replacement is itself a head for the merged line, so the extension also applies when the whole first line is replaced (and not when the replacement itself ends with a line break).

Tests: RemovalWindowIntoDecoratorTest — 6 cases covering delete/replace windows ending inside a decorator, replacing the break between bulleted/task items, renumbering after the merge, and the downstream insert-crash. All 6 fail on main. Full suite passes, JS/Wasm compile clean. In the seeded sweep (100 seeds × 250 ops) these two holes accounted for every remaining crash; none remain with the fix.

[Copilot]

Pull request overview

Fixes a corruption/crash class in multi-paragraph delete/replace operations where a removal window can partially consume (or stop exactly at) a following list item’s decorator, leaving decorator fragments mid-line and causing later offset-based edits to crash.

Changes:

• Compute isLastDecoratorPartiallySelected using document offsets (not piece/buffer offsets) so partial-decorator swallowing works even when decorators live deep in the ADDED buffer.
• Add TextUpdateTransaction.extendPastOrphanedDecorator to mirror the delete-path behavior: when a replace consumes a terminating line break and stops at the next list item start, extend one char into the decorator and re-dispatch so existing logic swallows/renumbers correctly.
• Add RemovalWindowIntoDecoratorTest covering delete/replace windows reaching into decorators, break-replacement merges for bullet/task items, renumbering, and the downstream insert-crash regression.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
shared/src/commonTest/kotlin/com/jjrodcast/textkit/RemovalWindowIntoDecoratorTest.kt	Adds regression tests ensuring decorator fragments are not left mid-line after multi-paragraph delete/replace and that export/merge behavior remains stable.
shared/src/commonMain/kotlin/com/jjrodcast/textkit/editor/core/transactions/text/TextUpdateTransaction.kt	Extends replace windows into following decorators in the specific orphaning case, and fixes partial-decorator detection to use document coordinates.
shared/src/commonMain/kotlin/com/jjrodcast/textkit/editor/core/transactions/text/TextDeletedTransaction.kt	Fixes partial-decorator detection to use document coordinates, preventing missed “swallow decorator” behavior when buffer/document offsets diverge.

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[LaatonWalaBhoot]

@jjrodcast I think it can be merged