feat(templates): add the structured proposal document model

[DemchaAV]

Why

The proposal family's data layer carries one shape — ProposalData's titled prose sections with a flat timeline and pricing list. The structured business proposal look (brand marks, an authored multi-line title, an at-a-glance fact card, goal cells, a numbered scope list, authored deliverable columns, a phase grid with its own headers, priced rows with roles, a signing card) cannot be represented in it — and both ProposalData and ProposalDocumentSpec are records, so the existing model cannot grow those structures compatibly.

What changed

• templates.data.proposal gains a second, structured document model beside the narrative one: StructuredProposalData (fluent builder, 12 components) wrapped by the thin StructuredProposalDocumentSpec carrier (from()), plus the section records — ProposalBrand, ProposalTitleLines (lead/second/third), ProposalMetaLine, ProposalSummaryBlock, ProposalGlance (+Fact with icon/label/value/optional note), ProposalGoals (+Goal), ProposalScope (+numbered Item), ProposalDeliverables (two authored columns), ProposalPhaseGrid (+Phase, headers as an authored list — the column set belongs to the preset that draws the grid, unlike ProposalInvestment, whose two columns are fixed by the concept and therefore named fields), ProposalInvestment (+Row with a Role: NONE / SUBTOTAL / OPTIONAL — named for what the row is, not how it looks — and a total band), ProposalTermsBlock, ProposalAcceptance (statement + signature field labels).
• The near-duplications with the narrative records are deliberate and each is justified in Javadoc: ProposalScope.Item vs ProposalSection (numbered title+description row vs titled prose run), ProposalPhaseGrid.Phase vs ProposalTimelineItem (number/focus/output columns + grid-owned headers vs phase/duration/details triple), ProposalInvestment.Row vs ProposalPricingRow (row roles + separate total band vs flat list).
• Family conventions hold: compact-constructor normalization (null → "" / List.of(), Role.NONE), List.copyOf freeze on all 11 collections, no rendering/DSL/Jackson dependencies, every public member documented, largest file 247 LOC. The section records construct positionally; the top-level builder is where a document is assembled. package-info now names both models and which presets consume which. The narrative model is untouched.
• Consumers arrive separately: the first structured preset (the ported northline-proposal look) follows in its own PR, with a second variant of the same look behind it.

Verification

./mvnw -B -ntp verify -pl :graph-compose-templates → BUILD SUCCESS, 111 tests green, +7 new (StructuredProposalDataTest: empty-form normalization for all 12 components, null-string sweep across the leaf records and every section's heading/icon, Role default, frozen-collection sweep across all 11 lists, full builder pass-through, spec normalization + from()); javadoc:javadoc -pl :graph-compose-templates → 0 warnings. Additive public API only — no existing type changed.

Lane: canonical (templates.data.proposal) — additive data-layer API; no engine changes.