fix(engine): empty pageBackgrounds clears; row weights/children mismatch is IAE (PR-7.3)

[DemchaAV]

Summary

Two v1.6.5 stabilization fixes flagged by the extended audit
(2026-05-30), shipped together because both are P1 engine guards
sitting on the same DocumentSession lifecycle:

1. pageBackgrounds(emptyList()) now actually clears an earlier
   pageBackground(color) on the same builder. The Javadoc in
   GraphCompose.java
   promised that behaviour, but an
   if (!pageBackgrounds.isEmpty()) guard around the assignment
   silently treated the empty list as "leave the earlier value
   unchanged". The guard is removed.
2. Row weights / children size mismatch is now an
   IllegalArgumentException with a descriptive message, both in
   LayoutCompiler#distributeRowSlotWidths
   and in
   NodeDefinitionSupport#measureRow.
   The previous code walked off the end of the weights list with a
   raw IndexOutOfBoundsException whenever a caller bypassed
   RowBuilder and built a RowNode directly. RowNode's canonical
   constructor already validated this at construction time; the new
   engine guards are defence-in-depth for any path that bypasses the
   constructor (reflection-based deserialization, etc.).

This is the last of three short branches unblocking the v1.6.5 cut
(alongside PR-7.1 #80
ci/exec-plugin-drift-fix and PR-7.2 #79
chore/byte-buddy-test-scope). Tracked in the private
release-readiness taskboard.

Why

Both behaviours had Javadoc / engine-contract drift that would have
turned into user-visible bugs the moment a v1.6.5 user composed a
template with a default pageBackground(color) and then explicitly
cleared it, or built a RowNode programmatically. The audit caught
them before they shipped; cheap to fix now, expensive to fix after
the public release.

Test plan

New regression tests:

• PageBackgroundTest#pageBackgroundsWithEmptyListClearsEarlierPageBackgroundColor
  — builder pipeline pageBackground(LIGHT_GRAY).pageBackgrounds(List.of()) must emit zero page-background fragments.
• RowBuilderTest#rowBuilderWeightsMismatchAtBuildFailsWithDescriptiveMessage — new RowBuilder().weights(1, 2).addParagraph(...) x 3 .build() throws IllegalStateException mentioning both weights and children.
• RowBuilderTest#rowNodeConstructorRejectsWeightsThatDoNotMatchChildren — direct new RowNode(..., weights=[1,2,3], children=[a,b], ...) throws IllegalArgumentException with both sizes named.

Suite:

• ./mvnw test -pl . → 1019 tests, 0 failures, 0 errors (~45 s, locally on JDK 17).

• New tests pass locally

• Full canonical suite green locally

• CI green on PR (Guards / JDK 17/21/25 / Examples Smoke)