[raiseCatError]

Based on current dev (all of #489–#496 are merged).

• F3 (or H from the read-only views) shows a Markdown document rendered, except the block under the caret/selection, which shows its exact source and is fully editable. Another presentation of the one TextBuffer: every key is the editor's, switching views never changes the text.
• Built on the shared source map (Block.from/to, DocumentIndex): smallest safe region (whole list / table / fence), raw fallback for anything the parser cannot place, viewport anchored to a source line so revealing a block does not move what is above it, no re-parse on cursor movement, only changed items re-rendered, only visible rows drawn.
• Works with snippets, list/table helpers, undo/redo, recovery drafts, hunk review, Save As/Playground and heading navigation; in the split (F2) F3 switches the text pane between source and hybrid.
• Not in this PR (ledger): images in place, visual-line Up/Down, pointer input, tables as a grid, HTML files.

Tests: hybrid.test.ts (21, incl. a 300-step mode/move/resize walk asserting the text never changes, budgets on 4,000 lines, NO_COLOR/hostile/CRLF/BOM, three real-app tests). Combined run with workspace/split/editor/input-ownership suites: 180 pass. Independent review (separate Opus reviewer, adversarial): lossless-source invariant held through 150 randomized 400-step runs; nine real findings (stray-CR span shift, double caret at wrap boundaries, wheel undone, unbounded cache, per-key re-wrapping, stale cache key, tiny viewports, reload dropping hybrid) all fixed with regression tests in this PR. Not run: full suite, physical QA.
Refs #460.