feat: extend existing entity tables via #[Table(extends:)]

[markshust]

Summary

• Any module can add columns to another module's entity table without touching the original entity class
• Declare #[Table(extends: ParentEntity::class)] on a plain Entity subclass — its columns/indexes/foreign-keys merge into the parent's table schema
• The extender hydrates from the same row as a companion on the parent; Repository::save() persists parent + companion data in a single INSERT/UPDATE
• Supersedes feat(entity-extension): add entity extension system #59 with a much smaller surface (~700 LOC vs ~3.3k) by reusing the existing Entity pipeline instead of building a parallel EntityExtension hierarchy

Highlights

• New Entity APIs: companions(), companion(class) (typed via @template), attachCompanion()
• SchemaRegistry::registerEntities() is two-pass: parses all, then merges each parent's table with linked extenders
• Loud validation at parse/register time for: chained extension, redeclared PK, autoIncrement on extenders, duplicate column/index names (both class-strings in the error message)
• Rolling-deploy safe: extender hydration silently skipped when columns aren't yet migrated
• Repository safety guards: rejects extender ENTITY_CLASS at construct; rejects insertBatch() when entities have companions attached
• migrate:diff sees the merged table — no extra code needed for schema migrations

Test plan

• 5136 tests pass, 0 failures (full composer test suite)
• 8 new task-level test files cover attribute, metadata, factory, registry, hydrator, repository, integration
• End-to-end integration test against in-memory SQLite proves: schema merge, single-INSERT/UPDATE, rolling-deploy skip, discovery integration, both safety guards
• Docs page updated with usage, rules, schema merging, and rolling-deploy behavior

Closes #63

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