Luccas/sd 2838 unify nested tables rendering

[luccas-harbour]

Summary

Continuation of the SD-2838 unification effort, focused on nested table rendering. The painter and the layout engine had grown two parallel implementations of cell-segment expansion, partial-row pricing, and table fragment height math — slightly out of sync, and both living inside large renderer.ts / layout-table.ts files. This PR collapses them onto a single source of truth in @superdoc/contracts and breaks the painter's table branch out into co-located modules.

Shared cell-slice + fragment-height helpers (contracts/)

• New table-cell-slice.ts (moved from layout-engine/): owns getCellLines, getEmbeddedRowLines, describeCellRenderBlocks, createCellSliceCursor, computeFullCellContentHeight, and computeCellSliceContentHeight. Exposed via the contracts barrel and marked internal in JSDoc.
• New table-fragment-height.ts: computeTableFragmentHeight() is the single formula for adding header rows + body rows (with partial-row substitution) + cell-spacing gutters + outer borders when border-collapse: separate.
• layout-table.ts and the DOM painter both call into these helpers; the painter's even-split height approximation for embedded rows is gone — slice heights now come from the same recursive walk the layout engine uses.

DomPainter.renderFragment (table branch) decomposition

Carved out of renderer.ts into painters/dom/src/table/:

• renderResolvedTableFragment.ts — the former renderTableFragment method plus its resolveTableRenderData helper. Takes renderLine, snapshot capture, frame application, and SDT helpers as injected deps.
• fragmentKey.ts — tableFragmentKey() with the partial-row-aware cache key.
• snapshot.ts — getTableSnapshotFlags() for the inTableFragment / inTableParagraph .closest() checks used during paint-snapshot capture.
• embeddedTableFragment.ts — createEmbeddedTableFragment, mapEmbeddedTableRowSlice, computeRenderedTableFragmentHeight, and getEmbeddedTableSegmentCount, replacing the duplicated segment counters, visible-height loops, and partial-row computation inside renderTableCell.

Each new module has a co-located test. renderer.ts shrinks by ~150 lines and no longer carries table-specific helper closures.

Bug fixes uncovered along the way

• fix(layout-engine): price embedded table slice height — partial embedded table slices now report their actual visible height instead of the full table height when computing whether they fit on the page.
• fix(layout-engine): price partial embedded table slices — pricing now uses the shared cell-slice cursor so deeply nested partial rows are sized consistently with the splitter's decisions.
• Removed a redundant table-cell-slice.ts shim in layout-engine/ now that contracts/ owns the implementation.

Tests

• New unit tests in contracts/: table-cell-slice.test.ts (132 lines), table-fragment-height.test.ts (51 lines).
• New painter tests: renderResolvedTableFragment.test.ts, embeddedTableFragment.test.ts, fragmentKey.test.ts, snapshot.test.ts.
• Expanded renderTableCell.test.ts covers embedded-row slice mapping.
• renderer-dispatch.test.ts updated for the new dispatch path.

[linear-code]

SD-2838

[chatgpt-codex-connector]

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[luccas-harbour]

I deleted codex' comment as it related to the removal of rendering of list-item fragments, which was handled in another PR.