fix(core): preserve fixed table width during resize

[leandrotcawork]

Summary

Preserve the total width of fixed-width (dxa) tables when resizing columns in the ProseMirror editor.

This PR keeps imported fixed-width Word tables stable during interactive column resize by materializing deterministic colwidth metadata, tracking fixed-width table intent through toProseDoc, and normalizing edited widths back to the original table total.

Problem

Imported fixed-width tables could drift after column resize in the editor:

• dragging one column divider changed the resized cell width
• but the table total was no longer preserved
• the remaining columns were not rebalanced back to the original fixed table width
• exported DOCX could therefore diverge from the original fixed-width layout authored in Word

This was especially visible on imported tables whose width was defined in dxa and should behave as a fixed total-width grid.

Root Cause

The editor had enough information to resize columns visually, but not enough persisted structure to normalize the table back to a fixed total width after the transaction:

• imported fixed-width cells were not consistently carrying ProseMirror colwidth metadata
• the resize path did not distinguish fixed-width (dxa) tables from percentage/auto tables
• after a resize, the transaction preserved the edited pixel widths but did not redistribute them back into a stable fixed-width table model for serialization/export

What Changed

• materialize colwidth metadata for imported fixed-width table cells in toProseDoc
• thread table width intent (dxa vs non-fixed modes) through table conversion
• persist data-colwidth on table cell DOM parse/render so PM resize state round-trips correctly
• add focused fixed-resize utilities to:
  • read first-row column widths
  • compute the target fixed table total
  • normalize edited widths while preserving the total
  • distribute twips proportionally for table grid serialization
• add a fixedResizeNormalizerPlugin that post-processes resized fixed-width tables and rewrites cell/table widths back into a stable fixed-width state
• suppress active-cell decoration while a column resize handle is active, avoiding conflicting resize/editor visuals

Scope

This PR is intentionally limited to fixed-width table resize behavior.

It does not change header/footer layout logic from #425, although both PRs touch some shared ProseMirror table conversion files.

Validation

Ran:

bun test packages/core/src/prosemirror/extensions/nodes/tableFixedResizeUtils.test.ts packages/core/src/prosemirror/conversion/toProseDoc.test.ts
bun x tsc --noEmit -p packages/core/tsconfig.json
git diff --check origin/main...

Result:

• tests passing
• core typecheck passing
• no diff whitespace issues

Notes For Maintainer

I kept this as a separate review slice from the header/footer work in #425 so the resize behavior can be reviewed on its own.

Functionally this patch applies cleanly on top of main; the connection to #425 is organizational rather than runtime-coupled.

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[jedrazb]

Hey @leandrotcawork ! are you planning to pick up the work?