feat(templates): keep ModernProposal section headings with their content across page breaks

[DemchaAV]

Why

ModernProposal emitted each flowing section heading as a bare addParagraph(...)
sibling to the body or table it introduces. When a heading landed near the bottom of a
page and the first slice of its content didn't fit below it, the heading rendered alone
at the page bottom while its content flowed to the next page — a boxed title stranded
from what it introduces (the "Investment" heading on page 1, its table on page 2).

What changed

• The four flowing headings in ModernProposal — the per-section loop title,
  Timeline, Investment, and Acceptance terms — now render inside their own
  SectionBuilder.keepWithNext() section instead of a bare paragraph, so each title
  binds to the first slice of the block it introduces and relocates with it across a
  page break. keepWithNext() binds a section to its following sibling, so the
  heading has to be its own section; the wrapper is zero-padding / zero-margin with the
  title paragraph as its single child, so placement is unchanged unless a heading would
  otherwise strand.
• No engine change — keepWithNext() already exists on develop; this is template
  wiring only.
• Other template families need no change and are left untouched: CV single-column
  presets are already wired (ModernProfessional, BoxedSections, ClassicSerif, …);
  the CV two-column presets and the invoice build their bodies as an atomic
  addRow(...) that never splits across a page, so a heading there cannot strand; the
  cover letter has no interior section headings.

Verification

./mvnw -B -ntp clean verify -pl :graph-compose-templates,:graph-compose-qa -am →
BUILD SUCCESS.

Focused: ./mvnw test -f qa/pom.xml -Dtest=ProposalHeaderKeepWithNextTest,ProposalV2VisualParityTest,ModernProposalSmokeTest
→ Tests run: 9, Failures: 0.

• New ProposalHeaderKeepWithNextTest: asserts the four title sections are
  keep-with-next (positive) and the body sections are not (negative — marking the body
  would bind the heading to the next block); an end-to-end test reproduces the
  template's separate-pageFlow().build()-group shape and asserts the title relocates
  to its body's page, with a control proving the title strands on the earlier page
  without the flag.
• ProposalV2VisualParityTest passes with no baseline re-bless — the wrapper is
  geometry-neutral on the canonical sample.

Lane: templates (canonical) — ModernProposal preset wiring; no engine or
public-API surface touched.