refactor(engine): retire the dormant ECS/Entity runtime

[DemchaAV]

Why

The render path — DocumentSession → LayoutCompiler → LayoutGraph → the
fixed-layout backend SPI → PdfFixedLayoutBackend — carries the whole engine, but
the legacy Entity-Component-System runtime (EntityManager, SystemECS, the
Entity component model, the ECS render pipeline) is dormant: nothing on that path
references it. It only stayed compiled because a handful of live value types and one
snapshot helper still linked against it.

What

• Removed the ECS cluster: engine.core.* (EntityManager / SystemECS / Canvas
  / …), the Entity component model with its geometry / layout.coordinator /
  renderable companions, engine.pagination.*, engine.render.* with the
  engine.render.guides renderers, engine.exceptions.*, and
  LayoutSnapshotExtractor — plus the unit tests and com.demcha.mock fixtures that
  only drove them (46 main + 13 test files).
• Decoupled the six live leaves that still compiled against it, byte-identically:
  • Margin / Padding / Anchor drop their ECS render methods and the
    RenderCoordinate interface; they stay plain value types (still carrying the
    empty Component marker) with every factory / accessor / equals/hashCode
    unchanged.
  • ParagraphWrapping / TextFlowSupport call TextControlSanitizer.remove(...)
    directly — the removed BlockText.sanitizeText was a pure delegate to it.
  • LayoutInsetsSnapshot inlines the snapshot-rounding helper it borrowed from
    LayoutSnapshotExtractor.
• Pruned the four test-logback loggers that named the removed packages and noted the
  removal in the 2.0 changelog. The four public snapshot records, the Component
  marker, Anchor / HAnchor / VAnchor, ContentSize, and the live
  engine.render.{word,pdf} trees are kept.

Tests

• ./mvnw -f aggregator/pom.xml verify -pl :graph-compose-qa -am — full multi-module
  compile + javadoc gate + the qa visual/parity byte-identity suite: 643 tests, 0
  failures. Rendered output (PDF pixels, layout snapshots, page margins) is
  identical, and the public guideLines(...) overlay is unaffected.
• A repo-wide grep across all modules confirms zero surviving references to any
  removed type.