- render(): drop the format-based dispatch escape hatch, reject a raster
  format override on document output, and give .svg its own clear error.
- slides property returns a copy so the Canvas type guard can't be bypassed.
- contact_sheet validates image paths up front for consistent errors.
- PPTX exporter no longer validates the first slide twice.
- Document that mixed-size decks clip larger later slides in PPTX.
- Add tests for render dispatch, to_pptx() bytes, and column clamping.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLVAwDncqBSdzXnid1YMFB

Add an optional default size to Deck (Deck(width, height) and
Deck.from_aspect_ratio) so slides can be written as bare Canvas() that
inherit it, instead of repeating dimensions on every slide. An explicitly
sized canvas keeps its own size.

Canvas now accepts being constructed without a size: it stays "unsized"
(accepting layer builders but refusing render/diagnose/serialize with a
clear error) until a size is assigned directly or injected when the canvas
is added to a sized deck. The Deck constructor takes width/height first;
pre-built canvases now pass via Deck(slides=[...]).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLVAwDncqBSdzXnid1YMFB

- Drop the dummy first-slide arg from the multi-canvas exporter calls by
  making PdfExporter/PptxExporter accept an optional canvas; single-canvas
  save()/export_bytes() now guard for it explicitly.
- Preserve a deck's default size and a deck-level theme across to_json/
  from_json; the shared theme resolves $theme.* tokens in every slide.
- Share one aspect_ratio_dimensions() helper between Canvas and Deck and
  raise ValidationError for malformed ratios.
- Pre-validate all slide assets before writing a raster sequence so a
  failing slide leaves no partial output on disk.
- contact_sheet() accepts an (R,G,B[,A]) tuple and validates the
  background eagerly instead of failing opaquely at render time.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLVAwDncqBSdzXnid1YMFB

The contact-sheet grid was speculative surface area: nothing in the
library, examples, or workflows used it, and a proof-sheet image is a
separate concern from the multi-page/multi-slide/sequence outputs the
deck exists for. Drop the method, its private helpers, the now-unused PIL
import, and the related tests and docs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLVAwDncqBSdzXnid1YMFB

Deck(slides=[...]) already covers seeding several slides at once and
.slide() covers appending one, so a separate variadic add() was redundant
surface area. Drop it and update tests and docs to use the constructor or
chained slide() calls.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLVAwDncqBSdzXnid1YMFB