v2(Phase 3): widgets/ layer — LEGO bricks for preset authors

[DemchaAV]

Summary

Closes the «пишу свой preset — выбираю кубики и вставляю» gap.
Until now, an author who wanted a section title style different
from BoxedSections' banner had to read another preset's private
rendering methods and copy-paste. With widgets, they pick a named
visual building block.

This is what gives v2 its «понятный, читаемый, переиспользуемый»
authoring story.

⚠️ Stacked on #47 and #48. After both merge, this PR's diff
shrinks to just the widgets-layer commit.

What changes for preset authors

Before — preset's compose() had ~30 lines of DSL plumbing or
called CV-specific renderers:

host.accentBottom(theme.palette().rule(), theme.spacing().accentRuleWidth())
    .padding(new DocumentInsets(8, 0, 2, 0))
    .addParagraph(p -> p
        .text(TextOrnaments.spacedUpper(title))
        .textStyle(theme.entryTitleStyle())
        .align(TextAlign.LEFT)
        .margin(DocumentInsets.zero()));

After — preset reads as a sequence of visual decisions:

SectionHeader.underlined(host, title, theme);

A full preset's compose() is now ~12 lines (was ~30). Reading it
no longer requires mentally executing DSL.

New: cv/v2/widgets/

File	Variants
Headline.java	.spacedCentered, .rightAligned, .render(align, spacedCaps)
ContactLine.java	.centered, .rightAligned, .render(align, order)
SectionHeader.java	.banner, .underlined, .flat(color)

Three layers of customisation per widget:

1. Convenience factory — one line, common case
2. Low-level render(...) — same widget, more knobs
3. Inline DSL — when no widget fits, bypass it; widgets are
   optional helpers, not required wrappers

Refactor — 3 presets use widgets

• BoxedSections: 3 widget calls in compose()
• MinimalUnderlined: 3 widget calls, private renderUnderlinedTitle deleted
• ModernProfessional: 1 widget call (SectionHeader.flat) plus
  renderHeader / renderContact kept inline because the
  preset-specific colours (slate-blue name, royal-blue links) don't
  fit widget defaults. This is the documented "widget doesn't fit
  → inline" pattern.

Backward compatibility

• components/HeadlineRenderer → @Deprecated, delegates to widget
• components/ContactRenderer → @Deprecated, delegates to widget
• components/BannerRenderer → @Deprecated, delegates to widget

Old v2 callers keep compiling unchanged.

Docs

• AUTHORS.md gains full "Widget cookbook" section: catalog
  table per widget, 12-line composing-a-preset example,
  when-to-add-a-widget guidance, "doesn't fit → inline" recipe,
  forward-looking examples (Badge, IconLabel, Divider — not yet
  implemented, included as illustration where the catalog can
  grow).
• package-info.java ASCII diagram updated with widgets/ layer
  between presets/ and components/.

What's NOT changed

• Engine — zero edits.
• v1 CV surface — zero edits.
• Visual output — all 3 v2 PDFs render pixel-identical to
  baseline. Refactor proven.
• Public v2 API — old *Renderer classes still work
  (deprecated, delegating); widgets are additive.

Test plan

• WidgetSmokeTest — 4 new tests cover every widget variant
• All prior v2 tests stay green (38 tests across data/theme/presets)
• mvn test — 889/889 pass
• cv-boxed-sections-v2.pdf — pixel-identical to baseline
• cv-minimal-underlined.pdf — pixel-identical to baseline
• cv-modern-professional-v2.pdf — pixel-identical to baseline
• CI green

What this PR completes (and what it doesn't)

This PR finishes the concept: 4 layers (data / theme /
components / presets) + widgets layer in between gives authors
both structural understanding ("what goes where") AND
compositional ergonomics ("pick a brick, place it").

This PR does NOT migrate v1 templates. That's a separate decision —
"when do we deprecate v1 and port the remaining 13 presets". With
this PR, the concept is sufficiently proven that we can make that
call from data, not theory.