fix(sdt): SDT field deletion and editing contract (SD-3218, SD-3165 and SD-3257)

[luccas-harbour]

Note

This PR fixes SD-3218, SD-3165 and SD-3257

Summary

Hardens the deletion / typing / caret contract for inline structured-content (SDT) fields so editing across the wrapper boundary behaves consistently with Word and stops leaking into locked content.

Stacked on top of luccas/sd-3237-bug-sdt-hover-and-click-to-place-cursor-interactions-are; review against that branch.

What changes

Inline SDT deletion across the wrapper boundary

• New commands selectInlineSdtBeforeRunStart / selectInlineSdtAfterRunEnd (packages/super-editor/src/editors/v1/core/commands/selectInlineSdtBeforeRunStart.js). When the caret sits at the start of the run after an inline SDT (Backspace) or the end of the run before one (Delete), the command installs a TextSelection over the SDT's inner content instead of letting run-aware deletion scan into the locked field.
• Wired into the Backspace and Delete keymap chains (keymap.js).
• For unlocked / contentLocked modes the next keystroke deletes the selected content; for sdtLocked / sdtContentLocked the command consumes the keystroke and the boundary stays put.
• Selection is applied uniformly across all lock modes as a TextSelection (not a NodeSelection over the chrome), so labels/handles never get caught in the selection. A SELECT_INLINE_SDT_BEFORE_RUN_START_META meta flag tells the structured-content select plugin not to collapse this selection back to a TextSelection over the wrapper. (SD-3165)

contentLocked wrapper deletion

The lock plugin previously promoted exact-content selections covering a contentLocked SDT to a NodeSelection, requiring a second keystroke to delete. It now deletes the wrapper directly on Backspace/Delete. Cut still promotes to NodeSelection so PM's clipboard handler can serialize the wrapper in one keystroke (structured-content-lock-plugin.js).

Empty inline SDT placeholder

Empty inline SDTs were filtered out of layout entirely, leaving a zero-width invisible wrapper with no caret target. Introduces a layout-only placeholder run that flows end-to-end:

• contracts: TextRun.visualPlaceholder = 'emptyInlineSdt' and an isEmptyInlineSdtPlaceholderRun guard. sliceRunsForLine preserves placeholders that have no chars.
• pm-adapter: emit a placeholder run for inline structuredContent nodes with empty/missing content. mergeAdjacentRuns refuses to merge placeholders into neighbors.
• measuring/dom: reserve an 8px inline box (0px for appearance='hidden') without taking the empty-paragraph code path.
• painters/dom: render <span class="superdoc-empty-inline-sdt-placeholder" aria-hidden="true"> inside the SDT wrapper and tag the wrapper data-empty="true". CSS gives the wrapper a visible border without inflating line-box height (styles.ts).
• DomSelectionGeometry: anchor the caret to the line's Y (the placeholder is height: 0) and always to its left edge.
• lock plugin: collapsed Backspace/Delete inside an empty inline SDT deletes the wrapper when the lock mode allows.

beforeinput insertText at inline SDT boundaries

When the caret sits directly before or after an inline SDT, native beforeinput handling can drift the inserted character into the wrapper. editable.js now forces the manual insertText path for collapsed selections at those boundaries so the character lands on the outer side.

sdBlockRev on ancestors of inline edits

Text changes inside an inline SDT weren't bumping the containing paragraph's sdBlockRev because nodesBetween over the replace range didn't always visit the ancestor block. block-node.js now walks the ancestor chain at each changed range's boundaries.

The plugin's own metadata transaction is tagged with BLOCK_NODE_METADATA_UPDATE_META so it doesn't re-trigger appendTransaction and the lock plugin doesn't filter it out.