[Platform] Add ModelCard with pricing, region and extra metadata to model catalog

[wachterjohannes]

Q	A
Bug fix?	no
New feature?	yes
Docs?	no
Issues	-
License	MIT

Adds an optional, objective metadata layer to model catalog entries so a Model
can answer "what do you cost?" and "where do you run?". This is the data layer that a
cost tracker (token usage → money) and a criteria router (filter by region/tier) build
on; both are out of scope here.

What's added

• ModelCatalog\ModelPricing — immutable VO with per-1M-token rates, cache-aware
  (cachedInputPerMillion / cacheWritePerMillion, matching TokenUsage's cache
  counters), currency defaulting to USD.
• ModelCatalog\ModelCard — immutable VO holding platform-blessed facts (pricing,
  region) plus an open, app-owned extra bag (get() / all()). Platform code never
  branches on extra contents.
• Model::getCard(): ?ModelCard — new optional 4th constructor slot, defaulting to
  null.
• AbstractModelCatalog parses an optional metadata key per entry and attaches the
  card.

Usage

// In a ModelCatalog's $models array:
'gpt-x' => [
    'class'        => Gpt::class,
    'capabilities' => [Capability::INPUT_TEXT, Capability::TOOL_CALLING],
    'metadata'     => [
        'pricing' => ['input' => 5.0, 'output' => 30.0, 'cachedInput' => 0.5],
        'region'  => 'us',
        'extra'   => ['tier' => 'flagship'],
    ],
],

// Consuming it:
$card = $catalog->getModel('gpt-x')->getCard();
$card?->getPricing()?->getInputPerMillion(); // 5.0
$card?->getRegion();                          // 'us'
$card?->get('tier');                          // 'flagship'

Notes

• Fully backward compatible — metadata is optional and getCard() returns null
  when absent; no existing catalog entry needs editing.
• The getter is named getCard() (not getMetadata()) on purpose: the platform already
  has a generic mutable Metadata\Metadata bag exposed via getMetadata(): Metadata on
  every Result/Message (runtime side-channel data). A ModelCard is a static catalog
  descriptor, so it gets its own name to avoid overloading that convention.
• Mechanism only — no real bridge pricing data is shipped; apps populate their own
  catalogs. Model subclasses that override __construct (Claude, OpenAi/Gemini/Scaleway
  Embeddings, Scaleway, Voyage) forward the new argument so the card isn't silently
  dropped; a regression test guards this through a subclass.

Possible follow-ups (separate PRs): wiring a pricing source such as ModelsDev,
populating real bridge data, the cost tracker, and the criteria router.