Region-scoped select-all and cross-host selection delete

[namedgraph]

Summary

Each block being its own contenteditable host meant the browser confined selections to a single block: Ctrl+A never escaped it, and a mouse sweep across blocks painted but could not be deleted. This PR gives the editor its own cross-host selection handling, Google-Docs style.

• Two-stage Ctrl/Cmd+A — stage 1 stays native (the browser scopes select-all to the focused host); once the host is fully selected (or empty), the next press selects all blocks of the region as a document-level range. Ctrl+A away from a caret — body focus, fresh load, a focused image island — selects the active region too (local:active-root() precedence); native page select-all only when no editable region exists. The selection never reaches the host page.
• One delete machine (src/select.xsl, dispatched from edit.xsl's host/body/image keydown) serves stage-2 selections and mouse sweeps alike. Block-granular, never one raw deleteContents: fully covered blocks removed whole; partial edge hosts trimmed via host-scoped sub-ranges; composites holding a range boundary keep their grid — covered cells cleared, flow cells collapse back to text hosts (B3/B4 doctrine) — while a fully covered composite goes whole; non-composite edge remnants merge Docs-style with the caret at the seam (never with pre); emptied containers pruned, chrome re-injected, an emptied region reseeded with a fresh paragraph.
• Region discipline — the range clamps to a single region (one gesture = one region-keyed undo entry); other regions stay byte-identical.
• Suppression — typing/Enter/Tab/paste over a cross-host selection are inert (type-to-replace is future work); Ctrl+C and plain arrows stay native.
• Help modal documents select-all, selection delete and undo.

Test plan

• New tests/browser/select.mjs (registered in run.mjs): both Ctrl+A escalation routes and the away-from-caret paths (body, engaged-region precedence, image island), sweep deletes with Backspace/Delete parity, seam-caret merge semantics, same-list item merge, composite partial/full coverage, cross-region clamping, gesture suppression — every mutating case checked against the I1–I5 invariants and an exact one-undo baseline restore.
• Full browser matrix (12 suites incl. invariants) and the headless suites pass; no extractor/canonical changes.

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