fix(render): lazy playwright-core — static import cost ~20s per omp startup

[rz1989s]

What

Fixes #21.

extensions/render.ts statically imported playwright-core. omp's guarded extension loader preloads every statically resolvable import of an extension module at host startup — measured ~20s for playwright-core's ~9MB graph (a native bun import of the same package: 0.57s). chromium is used only inside the execute path, so every omp start paid 20s for a browser driver that almost never launches.

Attribution (omp 18.1.12, Apple M5)

Config	omp total startup
all 3 plugins	~24–28s, loadExtensions warnings at 10s/20s
keystone disabled	5.5s
keystone skills-only	6.3s
playwright import line deleted	4.0s
import type (type-only) instead	still 27.7s — loader preloads type imports too
dynamic await import("playwright-core") in fn body	still 27.7s — loader preloads dynamic-import literals too
runtime-assembled specifier (this PR)	5.2s ✅

Change

const spec = ["playwright", "-core"].join("")
const mod = await import(spec)

Memoized (cachedChromium), loads once on first actual render. Comment block in the file documents why the indirection exists and forbids reintroducing any static reference (including import type).

Verification

• pnpm typecheck clean; full suite green: engine 162, lint 103, render 4 (real chromium launches through the lazy path), examples 1.
• Real render under bun (omp's runtime) with the indirected import: screenshot + computed styles + DOM snapshot all produced.
• omp -p startup timing: 27.7s → 5.2s, no Still starting warning.

Follow-up (out of scope here)

The omp-side loader behavior is the deeper bug: preloading extension import graphs (even type-only/dynamic) costs ~35× native import. Will report upstream to omp (can1357/tap, omp.sh).