TP-F2 — Theme marketplace metadata

[ZhuchkaTriplesix]

Epic: #159
Depends on: TP-03 (manifest model), TP-10 (ThemeDefinition), shipped in 0.4.2

Source: docs/planned-0.4.3.md, docs/market-tech.md

Goal

Extend querya.theme.v1 and ThemeDefinition with optional marketplace-oriented metadata so themes can later appear in an Extensions → Explore UI without another breaking schema change.

Files

• lib/core/theme/parser/querya_theme_manifest.dart
• lib/core/theme/theme_definition.dart
• lib/core/theme/theme_registry_service.dart
• docs/theme-custom-json.md
• test/fixtures/themes/ (samples with metadata)
• optional stub: lib/core/market/extension_manifest.dart (align field names only; no HTTP yet)

Implementation

Add optional manifest fields (backward compatible):

• author, homepage, license
• tags (string array)
• preview (relative asset path or HTTPS URL string — parse only; no remote fetch in this issue)
• description (short markdown/plain text)

Map into ThemeDefinition for picker display (subtitle, tooltip, future card layout).

Align naming with future ExtensionManifest from market-tech (id, name, version, type: theme, sha256Checksum stays install-time concern for TP-F4).

Update JSON schema docs and one fixture theme with full metadata.

Acceptance Criteria

• Old theme files without new fields still load unchanged.
• Registry exposes metadata on ThemeDefinition for UI consumers.
• Theme picker can show author or tags when present (minimal UI — badge or subtitle).
• Docs document every new optional field and defaults.
• No network calls in this issue.

Tests

• Parser accepts fixtures with and without metadata.
• Registry scan preserves metadata on ThemeDefinition.
• Regression: existing querya.theme.v1 fixtures still parse.

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