Rebuild examples as a generated symbology gallery

[s2x]

What

Replaces the seven tutorial scripts in examples/ with one generator: php examples/gallery.php, which builds a symbology gallery from whatever the registry supports.

• examples/index.md — every supported QR/barcode, one row each
• examples/codes/<symbology>.md — one page per code: payload, capabilities, and every renderer's output (SVG, PNG, HTML div/table, ASCII ×3); a renderer that cannot draw a symbol faithfully says so on the page instead of being skipped
• examples/assets/ — the image files the pages point at

The list of symbologies and renderers comes from the registry, so nothing in the generator hardcodes "every" — a symbology added to Defaults appears in the gallery without this file changing.

Why

The old examples were tutorial-shaped; what a person evaluating the library wants first is what does it draw. A gallery answers that per symbology and per renderer at once.

Gallery output is generated, never committed — a gallery is a claim about what the library draws right now, so it is regenerated the same way the tests are run. ExamplesTest executes the generator on every CI build and verifies its pages against the registry (every symbology linked from index.md, every renderer mentioned on every page), so coverage of the gallery is a fact, not a promise.

Verification

• composer lint (php-cs-fixer, PHPStan, Rector, kb-lint) — clean
• vendor/bin/phpunit tests/ExamplesTest.php — 4 tests, 280 assertions, OK
• composer validate --strict — valid
• All 30 symbologies render on all 7 formats at default options; no Compatibility refusals with the chosen payloads