Add Lambda Core in native HTML and CSS

[kserrec]

Summary

• add a self-contained Lambda Core implementation using only native HTML and CSS
• encode Church booleans through CSS branch selection and Church numerals through CSS counters
• demonstrate TRUE, FALSE, NOT, AND, OR, ZERO, SUCC, PRED, and ONE
• render the implementation in headless Google Chrome and compare its observed output with a checked-in baseline
• install poppler-utils in CI so pdftotext can observe the browser-rendered text

The implementation contains no JavaScript, SVG, external resources, generated source, or browser extension code.

Testing

• sh languages/h/html-css/test.sh
• sh run-tests.sh

Local harness result: PASS h/html-css; 10 passed, 0 failed, 6 skipped because their toolchains are unavailable locally, and 1 pre-existing Java baseline remains pending.