Export movie cells, size the exported figure box by its shape, and fix three report defects

[CSSFrancis]

Why

• Going through the report layer for the embed cleanup turned up three real defects and one missing export branch. None of them need the new embed contract, so they go first, on their own.
• A movie cell exported as nothing at all: no still, no caption, in static HTML, interactive HTML, the slides deck and the PDF. An unhandled cell type is indistinguishable from an empty report.
• Exported figures all got the same 480 px box whatever their shape, so a wide 1×3 row was letterboxed and a square pattern was stretched. The sidebar cell already sizes by aspect ratio; the export now does the same.

The three defects

• compose._finalize_edit was one of five copies of the per-cell teardown and the only one that never cleared the baked PNG, the offline flag and the photo bytes. There is now one ReportManager.drop_cell_resources, called from every per-cell site; _finalize_edit no longer leaks.
• The sidebar's copy of figurePayloadFromDrop lacked the peekWindowDrag() fallback the other two copies had, so a real window drag onto the body between two cells could resolve to nothing. One exported function in dnd.ts now, used by all three callers.
• The split cell's text pane was the markdown editor without its toolbar and without Ctrl-B / Ctrl-I. MarkdownPane is extracted from ReportCell and rendered in both.

Export changes

• Movie cells export as their poster still with a play badge and their caption; in interactive mode a rendered GIF or MP4 is inlined when it fits.
• EMBED_BUDGET_BYTES = 100 * 2**20 is the one budget for anything a report file inlines. Over it the cell exports as its still plus a note naming the size and the budget. The embed contract PR will move this constant to its home and reuse it.
• A figure with a caption but no pixels keeps the caption above an empty framed box instead of vanishing.
• The figure box is sized by aspect ratio like the sidebar cell, the iframe keeps the figure's natural size and is scaled into the box, and an embed that reports its own height overrides both. The slides deck gets the same box.
• The renderer's aspect constants and the Python ones are pinned against each other by parsing the TSX, and the drift test runs the same helper over an edited copy so it actually proves the guard bites.

Lines

• Part 1 (movie export, aspect box) adds about 290 production lines, nearly all in export_html.py. That is a missing feature, not duplication.
• Part 2 (the three defects) is net about −85 production lines: five teardowns became one, three drop readers became one, two markdown editors became one.

Tests

• Failing test first for each defect and for the movie export. Python: test_report_export.py (cell-kind coverage over model.CELL_TYPES, the figure box, the TSX guard, the embed height), test_report_compose.py (the teardown), test_report_present.py.
• e2e: report_export_movie.spec.ts (still + badge in static, inline GIF in interactive, the box sized past a fixed height, a blender embed filling its box) and compose_real_drag.spec.ts (a real titlebar drag onto the sidebar gap lands a cell; Ctrl-B in the split pane wraps the selection).
• Full Python suite 3476 passed; typecheck clean; the report specs 30 passed.

Changelog

• Five one-sentence fragments under upcoming_changes/, one per user-visible change.