feat: add elum-pre / elum-code primitives for #4

[truffle-dev]

Draft PR for #4. Opening as a draft so you can react to the issue first; happy to mark ready for review or close it depending on how you want the primitive shaped.

What this adds:

• pre.elum-pre for multi-line code blocks. Uses var(--elum-color-surface), var(--elum-color-border), var(--elum-color-fg), var(--elum-font-family). Horizontal scroll on overflow; no soft-wrap. tab-size: 2.
• data-language="bash" | "html" | ... on .elum-pre surfaces a small uppercase label in the top-left via ::before, reusing the same muted color and tracking recipe as .elum-toolbar-label. Decorative attribute, not announced.
• code.elum-code for opt-in inline runs that need a visible edge (border + small radius). Plain <code> continues to inherit from base.css for in-prose runs that should blend.

Wired through:

• packages/core-css/src/components/code.css (new file, 51 lines).
• packages/core-css/src/index.css (one new @import).
• packages/core-patterns/snippets/index.html (one inline <code class="elum-code"> example plus two <pre class="elum-pre"> blocks, one with data-language).
• examples/playground.html (new Code card in a full-width grid row between Rows and State Matrix so all three themes are reviewable in one screenshot).
• docs/component-usage.md (new Code Block section).
• tests/core-css-contract.test.mjs (extends bundle list, extends entrypoint-imports test, adds elum-pre and elum-code to the public-class assertion, and adds a focused test for the scroll behavior and attr(data-language) hook).

Verification:

• npm test passes all 23 + 1 new tests.
• Manually walked the playground in iron, neon, and dust at desktop and narrow widths. The Code card sits cleanly with the other tiles in all three; the language label reads as part of the kit, not a foreign affordance.
• Worked example using the same primitive on a real page: https://truffle.ghostwright.dev/public/glyph-nook/ (toggle theme top-right; scroll down on the Glyph and Built panels to see where a code block would naturally drop in).

Notes for review:

• I left the [22] components count in examples/playground.html alone since that copy is hand-curated; happy to bump to 24 in a follow-up if you want it included here.
• The tab-size: 2 is a small opinion; happy to drop it or expose it as --elum-pre-tab-size if you'd prefer the kit stay neutral.
• The two adjacent primitives I mentioned in v0.2 candidate: elum-pre / elum-code primitive for multi-line code blocks #4 (elum-kbd, elum-spinner) are not in this PR. Glad to send separate scoped PRs for either if there's interest; this one keeps the diff small.