refactor(painters/dom): unify SDT container rendering (SD-2838)

[luccas-harbour]

Summary

Consolidates SDT (Structured Document Tag) container rendering across paragraphs, tables, and nested tables into a single shared module, then fixes a long tail of nested/split-fragment chrome bugs that the unified pipeline made tractable.

Previously, the duplicate-suppression contract was implicit: cells were handed a raw tableSdt reference and each renderer reimplemented its own metadata comparisons. With nested tables, idless SDT controls, and split fragments, this produced missing borders, duplicated chrome, clipped labels, and chrome that disappeared across page splits. The new pipeline uses a key-based ancestor chain and an upward chrome-rendered callback, so every renderer (paragraph, table fragment, table cell, embedded table) makes the same decisions from the same data.

Changes

New shared module — painters/dom/src/sdt/container.ts (replaces utils/sdt-helpers.ts)

• Renames applySdtContainerStyling → applySdtContainerChrome
• Adds shouldRenderSdtContainerChrome, getSdtContainerKeyForBlock, getSdtSiblingBoundaries
• Adds SdtAncestorOptions type — ancestorContainerKey(s) + ancestorContainerSdt(s) chains for explicit suppression

Contracts (contracts/src/sdt-container.ts) — new file

• getSdtContainerKey, getSdtContainerMetadata, hasExplicitSdtContainerKey, isSdtContainerMetadata
• layout-resolved drops the local sdtContainerKey.ts and imports from contracts (single source of truth)
• idlessSdtContainerKeys WeakMap groups id-less sibling controls by identity, with an AIDEV-NOTE explaining why alias/title matching is unsafe

Renderer threading

• renderTableFragment, renderTableRow, renderTableCell, renderEmbeddedTable, and renderParagraphContent all accept the ancestorContainerKey(s)/ancestorContainerSdt(s) chain and an onSdtContainerChrome callback
• Cells/tables flip overflow: visible only when a descendant actually renders chrome, not preemptively based on metadata
• sdtBoundary start/end flags are now correctly narrowed at split points (paragraph mid-line splits, embedded table mid-row splits)

Nested + split-fragment fixes (each is its own commit)

• Continue SDT chrome across paragraph splits, partial embedded tables, and nested-table continuations
• Suppress duplicate chrome when a nested table or cell already inherits the same container
• Preserve the SDT ancestor chain through nested tables so id-less controls don't re-render
• Skip media (drawings) at SDT boundaries
• Use the rendered lock mode (not the resolved one) so the dataset matches what's visually shown
• Allow nested SDT labels to overflow rather than being clipped
• Merge idless SDT siblings into a single container instead of fragmenting

Tests

• New contracts/src/sdt-container.test.ts (42 lines) and painters/dom/src/sdt/container.test.ts (200 lines)
• renderTableCell.test.ts expanded with ~1.2k lines covering nested SDT cases
• renderTableFragment.test.ts adds ~195 lines for ancestor/chrome-callback wiring

[linear-code]

SD-2838