fix(runtime): preserve focus and bound markup, accessibility, and dev-loop work

Large native applications can lose editable focus during view reflow, exhaust stack space while constructing markup with large message types, and spend unnecessary work on unchanged fragments and retained rendering data. This preserves focus and authored scroll intent, bounds markup construction and accessibility publication, and reduces idle and repeated rendering work.

The original 17-commit series is retained in order. Review groups:

• Focus, nested scrolling, accessibility and caret scheduling, including the matching continuity fixture updates.
• Reference and owned markup allocation/lifetime fixes, child-slot reservation, large-message regressions, and typed native drag context.
• Fragment watch caching and per-app capacity, retained path/layer indexing, streamed text-layout parity, and SQLite path ownership.
• Replay allocation and Node view-harness repairs needed by the complete tested tree.

Validation: exact head 9d526ebd96aa03a7b2b9d5c7e3cebec25f6dc44c passed all 29 jobs in fork CI 40078085646, covering core shards, examples, platform/WebView/GPU and scaffolding checks. Source ancestry, the complete 56-file delta and whitespace were checked again before this delivery; no source changes or local rebuild were made. The existing exact-head CI is reused for this review; newly triggered upstream checks are separate evidence. The local scripts/gate.sh fast was not rerun for this contribution delivery. Upstream CI, maintainer review and integrated application acceptance remain pending, so this PR is draft.

This replaces the incomplete delivery represented by closed #748, whose last head contained only the first five commits. It leaves that PR closed. Text Undo routing remains in #753; its two changed paths do not overlap this series. The combined behavior with that separate change has not been accepted in a real application, and the 29-job result does not cover that combination.