Show discreet PHP support bars in library roadmap timelines

As a library roadmap reader,
I want discreet PHP version support bars alongside library release and support bars,
so that I can compare their schedules while keeping the library roadmap visually dominant.

Context

Parent Epic: #125. Extends the timelines introduced by #126 and #127.

PHP lifecycles provide useful context for library planning. They must remain a secondary visual reference:
the LongitudeOne library roadmap is the main content. The only visible PHP-specific text is a version label such as PHP 8.4.

Acceptance Criteria

• A shared config/roadmaps/PHP.json configuration defines the PHP versions and their explicit release and support dates using the same configuration model as existing roadmap JSON files.
• Each PHP version configured with a period intersecting the display window is represented by one discreet bar in each configured library roadmap timeline, on the same date axis and shared display window as the library bars.
• PHP active-support and security-support periods can be distinguished while remaining visually subordinate to the library roadmap.
• PHP bars form a compact secondary group after the library bars, using thinner bars and muted styling; library bars retain their visual prominence and usable space.
• The only visible PHP-specific text added is each version label, formatted as PHP 8.4, PHP 8.5, etc.
• No PHP commentary, explanatory paragraph, section title, legend text, date label, status label, tooltip commentary, or PHP support table is added.
• The secondary bars remain readable and discreet in light and dark themes and at mobile widths.
• The generated SVG included in website HTML and portable Markdown shows the same PHP reference bars.
• PHP configuration is treated as shared reference data rather than a documented LongitudeOne library: no PHP landing page, documentation-version list, or library-navigation entry is required or generated.
• An unestablished library roadmap retains its existing fallback behavior; PHP data alone does not create a standalone roadmap for that library.

Constraints

• Preserve the exact filename PHP.json and reuse the existing roadmap model, validation principles, and build-time SVG generation.
• Use explicit dates sourced from the official PHP project; do not infer PHP support durations from LongitudeOne policies or alter library support dates.
• Keep detailed accessibility metadata non-visible; the understated styling must preserve readable labels and distinguishable periods.
• No client-side dependency or MDX component is introduced.

Definition of Done

The project's common Definition of Done applies.

• Tests cover multiple configured PHP versions, shared-axis alignment, display-window clipping, configuration validation, and unchanged library fallback behavior.
• Output checks verify that only PHP version labels are added as visible PHP text and that no PHP table or navigation entry is generated.
• Representative timelines are visually reviewed in both themes and at mobile widths to confirm that library bars dominate.
• Maintainer documentation explains how to maintain PHP.json and records official sources for its dates.
• npm test, npm run build, and applicable Markdown lint checks pass.
• composer test-local passes if defined; otherwise record it as not applicable to this Node.js documentation repository.
• composer quality passes if defined; otherwise record it as not applicable to this Node.js documentation repository.