[#15] Mock asset foundation: TDD plan + Assets service scaffold

[joshuapease]

First PR in the Mock Asset feature series (#15). This is the standalone
foundation the plan designates to land before feature work, plus the TDD
rework of the plan itself.

What's here

Plan rework (docs/plans/2026-05-25-feat-mock-asset-service-plan.md)

• Restructures the 4 phases into 12 test-first implementation units (U1–U12)
• Adds a Testing Strategy section (unit/functional split, harness affordances,
  the few manual-only items: Imager X Pro AC8, Imagick-absent NFR4)
• Maps every AC/NFR to a concrete Codeception test file
• Reframes Phase 1 as greenfield scaffolding — an audit found the feature is
  entirely unscaffolded (no WIP MockAsset to "fix" as the original implied)
• Notes ext-imagick is already in CI's PHP_EXTENSIONS, so generation tests
  need no workflow change

U1 — Assets service + plugin wiring

• Registers an assets component with a typed getAssets() getter and
  @property-read, establishing the convention that every component gets both
• Adds the make() seam returning a MockAssetBuilder (a minimal stub here;
  fluent setters / configure() / defaults arrive in U3, config application
  and URL signing in U5)

Testing

• ddev test unit — 13 tests, 34 assertions pass (4 new wiring/seam tests,
  written test-first; 9 pre-existing, no regressions)
• ddev phpstan — clean (level 4)
• ddev composer check-cs — clean (ECS)

Scope notes

• Deliberately foundation-only; U2+ (ext-imagick, builder, element, controller,
  Imager X) land in stacked follow-up PRs.
• MockAssetBuilder is intentionally an empty stub at this stage.