Split figma-transformer to Packagist so consumers stop vendoring the monorepo

## Problem

`php-transformer` was split to Packagist in #802 with `.github/workflows/php-transformer-split.yml` and a `.gitattributes` that keeps the dist lean. `figma-transformer` never got the same treatment. It has a package-root `composer.json` (`psr-4` → `src/`) but **no `.gitattributes` and no split mirror**, so there is no published package to install.

Consumers therefore vendor the entire monorepo. `static-site-importer/composer.json` declares an inline `type: package` entry pointing at `https://github.com/Automattic/blocks-engine/archive/refs/tags/figma-transformer-v0.1.3.zip` with `psr-4` rooted at `figma-transformer/src/`.

## Measured cost in a shipped product

The published `static-site-importer` v1.8.1 release asset is **8,191,363 bytes** (6.77MB of compressed entries, 28.36MB expanded, 3,080 files). Breaking the archive down by compressed size:

```
vendor/automattic/blocks-engine-figma-transformer 5077.3KB 2091 files 75.0% of vendor payload
vendor/automattic/blocks-engine-php-transformer 760.1KB 261 files
vendor/league/commonmark 241.4KB 382 files
```

Inside the figma vendor directory:

```
fixtures/ 3095.5KB 1067 files <- Figma test fixtures
php-transformer/ 764.6KB 448 files <- complete duplicate copy, including its tests
packages/ 456.8KB 392 files <- the JS engine
figma-transformer/ 704.8KB 154 files <- the only part anyone installs it for
docs/ scripts/ tests/ CONTEXT.md CONTRIBUTING.md ~54KB
```

**~4.37MB of the 6.77MB compressed package — 63% — is Figma fixtures, a duplicate php-transformer tree, and a JavaScript engine.** The duplicate php-transformer copy is 448 files against the real package's 261, because it predates the #802 export-ignore and still carries `tests/`.

What the consumer actually needs from all of it is one class. The only `FigmaTransformer\*` reference in SSI's runtime PHP is `Compression\ZstdCommandDecoder`, in `includes/class-static-site-importer-figma-import.php`.

## Scope

- Add `figma-transformer/.gitattributes` mirroring the php-transformer one: export-ignore `tests/`, dev tooling, `homeboy.json`, `composer.lock`.
- Extend the #802 subtree-split mirror to `figma-transformer`, publishing `automattic/blocks-engine-figma-transformer` from a split repo so the package root is `figma-transformer/` rather than the monorepo root.
- Let consumers drop the inline `repositories` entry and require the real package.

## Acceptance

- The installed vendor directory contains figma PHP source plus `docs/contracts` only — no `fixtures/`, no `packages/`, no nested `php-transformer/`.
- `git archive` of the split prefix carries no test or fixture trees.
- The figma import path still resolves `Compression\ZstdCommandDecoder` and the fig fixture e2e passes.
- Downstream, the `static-site-importer` release asset drops below 3MB.

## Note for the consumer migration

`static-site-importer.php` already probes both layouts when bootstrapping — `vendor/automattic/blocks-engine-figma-transformer/figma-transformer/figma-transformer.php` and `.../blocks-engine-figma-transformer/figma-transformer.php`. The split package shape is already anticipated by the loader, so the consumer side should be close to a dependency swap.

Downstream tracker: Automattic/static-site-importer#1397. Sibling packaging cleanup for php-transformer: #1359.

---

*AI assistance disclosure: this issue was investigated and drafted by Claude Sonnet 4.6 running in Claude Code, operated by @chubes4. The AI inspected `blocks-engine` at `trunk` (`f8e12d04`), `static-site-importer` at `main` (`52d589f`), and the published v1.8.1 release asset, and computed the archive breakdowns quoted above by reading the ZIP central directory. Reviewed by a human before posting.*

Compare figma-transformer/composer.json and its future .gitattributes with the php-transformer equivalents, then read .github/workflows/php-transformer-split.yml and the split-prefix setup from #802. Check the static-site-importer loader and includes/class-static-site-importer-figma-import.php, then verify the split archive excludes fixtures and tests, the Figma import e2e passes, and the downstream release asset is below 3MB.

Written by the indexing model from the issue text.