fix: two defects the checkout example surfaced

[vicmaster]

Two of the three findings from building the v2.1.0 commerce example. Both exist because that example was built against the published package rather than the working tree. The third — the eyebrow tell counting form labels — is deliberately not here; it changes evaluator behaviour and deserves measurement rather than a guess.

1. Stamping a page structure erased the canvas genre

canvas_set_genre writes metadata.provenance.preset. applyStructure then recorded its own provenance by replacing that object, so declaring commerce and then stamping a layout dropped the genre entirely.

That is the order the docs encourage — declare what the screen is, start from a vetted pattern — and it failed silently. Nothing errored. The next evaluation simply reported genre.active: null and flagged all six money figures on the checkout as fabricated data, which cost a full evaluate cycle to notice and diagnose.

apply_preset has always merged into the existing provenance here. applyStructure now spreads it the same way, so a genre and a structure coexist rather than the later write winning.

2. export could not capture a design taller than its artboard

Phase 29 slice E taught screenshot to do this and left export behind, so saving a long design still meant working out the right height by hand — which is exactly what I had to do to produce the checkout screenshot for the README.

The underlying reason the two drifted is that each carried its own copy of the render setup. The viewport-expansion step is now a single shared helper used by both, so they cannot disagree again, and export takes the same fullPage option. It is ignored for PDF, which paginates on its own, and when specific nodeIds are requested.

The helper keeps the original explanation of why Puppeteer's own fullPage is not used here: it returns the full document on a fresh browser, then silently returns viewport-sized output once any non-fullPage capture has run in the same browser — which, with one browser held for the whole session, is the normal case rather than the edge.

Both tests were checked against the unfixed code

A regression test that would not have caught the bug is worth nothing, so I reverted the source and ran them:

• without fix 1, test-set-genre reports preset=undefined and the evaluator's genre as null
• without fix 2, test-render-capture exports 800x600 where the content is 1500px tall

The genre test asserts the end effect — that the evaluator still sees commerce — rather than only the metadata shape, since the metadata is a means and the evaluation is the thing that actually broke.

Verified

npm run test:fast 68/68, npm run test:full 100/100, discoverability 117/117. fullPage on export is documented in the README options table and in GUIDELINES alongside the screenshot guidance, so the two read as one option rather than two.