feat: make portfolio content crawlable + add SEO structured data

The entire portfolio rendered only on the PixiJS canvas, so crawlers saw a
near-empty body and the site ranked poorly for non-personalized searches. Fixes:

- NEW SeoContent.svelte: screen-reader/crawler-only (.sr-only) semantic HTML
  (h1 + bio + contact + Experience/Projects/Skills/Education), generated from
  data.ts (single source of truth, reuses aggregateSkills). Prerenders into
  build/index.html — verified it now contains the job/project/education text
  that was previously canvas-only. Also a big screen-reader accessibility win.
- JSON-LD schema.org Person in +layout (name, jobTitle, alumniOf, knowsAbout,
  sameAs LinkedIn/GitHub) — strong entity signal for name searches.
- Canonical URL; static/sitemap.xml + robots.txt Sitemap reference.
- Keyword-tuned <title>, description, og/twitter titles (name + role + stack).
- Demoted the transient welcome <h1> to a <div> so SeoContent's <h1> is the
  single canonical heading. Bumped SW cache to v2.

.sr-only utility added to app.css. check/lint/test/build clean; verified no
visual regression and no runtime exceptions (background + canvas unchanged).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app.css

@@ -31,3 +31,19 @@ body {
 	overflow: hidden;
 	background: var(--bg-space);
 }
+
+/*
+ * Visually-hidden but available to screen readers and crawlers. Used for the
+ * text version of the portfolio (the interactive content is canvas-rendered).
+ */
+.sr-only {
+	position: absolute;
+	width: 1px;
+	height: 1px;
+	padding: 0;
+	margin: -1px;
+	overflow: hidden;
+	clip: rect(0, 0, 0, 0);
+	white-space: nowrap;
+	border: 0;
+}