chore(engine): deprecate the legacy ECS engine packages (F11)

[DemchaAV]

Finding 11 — mark the dead legacy ECS engine deprecated

engine.core, engine.layout (+ engine.layout.container), and engine.pagination are the original Entity-based layout/pagination engine. The audit flagged them as a parallel second engine that no public API reaches:

• the canonical pipeline (GraphCompose.document() → DocumentSession → LayoutCompiler → LayoutGraph → PdfFixedLayoutBackend) imports nothing from them (verified);
• the legacy GraphCompose.pdf(...) / PdfComposer / com.demcha.templates.* surface has already been removed — so they're reachable only from the legacy engine regression tests (64 test files).

Their package docs actively misdirected contributors ("shared layout systems", "the normal PDF pipeline").

Change

• Package-level @Deprecated on the 3 pure-legacy packages (+ container subpackage) and rewritten, accurate package docs pointing at com.demcha.compose.document.layout.
• Package-level specifically so there is no deprecation-warning cascade across the 64 legacy test files — confirmed empirically: compile is clean, zero warnings reference these packages.
• The genuinely shared engine packages (engine.components, engine.measurement, engine.font, engine.render) are not deprecated, and are explicitly called out as such in the corrected docs.

Scope note

engine.render.pdf is intentionally out of scope: it's a canonical-mixed package (PdfFont / GlyphFallbackLogger + handlers / helpers subpackages) with simple-name collisions vs document.backend.fixed.pdf (PdfRenderSession etc.). Deprecating its legacy ECS classes cleanly needs a package split first — a follow-up.

Verification

Full verify green: 1152 tests, japicmp binary-compatible (adding @deprecated is compatible), architecture guards + visual-regression all pass. No runtime or behaviour change.