feat(page): normalize the authoring API — positional Header title, no more parens gotcha

[mhenrixon]

Closes #10. Part of #8 (Phase 1).

Problem

The kit's constructor conventions were inconsistent and the inconsistency was user-facing:

• Section.new(title, …) / Code.new(source, …) took their main argument positionally, but Header.new(title:, …) required a kwarg — DocsUI::Header("My guide") raised ArgumentError.
• Prose / Example take no positional args, so DocsUI::Prose do … end is a Ruby SyntaxError (it parses as a constant reference). The gem papered over this with a whole "the one syntax rule that bites everyone" docs section.

What changed

One convention: the primary argument is positional; modifiers are keyword arguments.

Before	After
DocsUI::Header(title: "T", eyebrow: "E")	DocsUI::Header("T", eyebrow: "E")
DocsUI::Prose() do … end	prose do … end (page helper)
DocsUI::Example() do |ex| … end	example do |ex| … end (page helper)

• Header takes a positional title — initialize(title = nil, eyebrow: nil, **opts). The legacy title: kwarg still works (silent compat, no deprecation warning); the positional wins if both are given. Existing consumer sites that call Header(title:) keep rendering verbatim.
• New lowercase page helpers prose / example (alongside the existing md). A lowercase method takes a block without parens, so the parens-with-blocks trap structurally cannot occur. The kit forms DocsUI::Prose() / DocsUI::Example() stay valid forever — nothing is removed.
• The helpers live in a new DocsUI::PageHelpers module mixed into DocsUI::Page. Extracting them into a module (rather than defining them inline on Page) makes them unit-testable against a bare Phlex host — DocsUI::Page includes Phlex::Rails::Helpers::Routes and cannot load in the standalone suite.
• The gem's own docs pages adopt the helpers; the dedicated "parens gotcha" section shrinks to a footnote. README gains an authoring convention section.

Why a module instead of inline methods

DocsUI::Page references Rails.application.routes at class-load time (via the Routes helper), so it cannot be loaded in the gem's Rails-free spec suite. A global Rails stub was ruled out — two code paths (icon.rb and the install generator) branch on defined?(Rails) and specs depend on Rails being absent under the random-ordered suite. Extracting the helpers into a module tests the real helper bodies without loading Page and without global-state pollution. (It also fixed a latent weakness in markdown_spec that was re-implementing md rather than exercising it.)

Test coverage

• spec/docs_ui/header_spec.rb — positional title, eyebrow, lead block; legacy title: kwarg renders identically to the positional form (backwards-compat proof); positional wins when both are given.
• spec/docs_ui/page_helpers_spec.rb — prose / example / md render the Prose wrapper / Example tabs / Markdown, exercising the real PageHelpers module through a bare Phlex host.
• spec/docs_ui/markdown_spec.rb — the md delegation test now includes the real module.

header.rb and page_helpers.rb are at 100% line coverage.

Verification

• bundle exec rake — rspec 103 examples, 0 failures; rubocop 42 files, no offenses
• Grep gate: no live DocsUI::Prose() / DocsUI::Example() in the gem's docs pages (only code { "…" } API references remain, which document that the kit form stays valid)
• Backwards compatible: Header(title:) covered by spec; no consumer-site call site changes required
• No new emitted CSS classes (Prose/Example classes already scanned) — no Tailwind @source change needed

Out of scope (per the issue)

• Renaming any component or removing the kit-form calls (DocsUI::Prose() stays valid).
• Callout / OnThisPage signature changes.