docs: elevate the public README experience - #16

Summary

Elevates the QuantumD repository landing page into an inclusive public
navigation and onboarding experience.

Adds:

• a branded accessible hero graphic
• role-based routes for new readers, quantum developers, security engineers,
  reviewers, auditors, educators, researchers, contributors, and security
  researchers
• a clear explanation of why QuantumD exists and what it proves
• the exact safe TestPyPI installation and checksum path
• accessible Evidence Graph and trust-boundary visuals
• a complete documentation map
• inclusive alpha-evaluation guidance
• explicit alpha limitations, security routes, and release provenance

All visual assets are repository-owned SVG files with titles, descriptions,
alternative text, and equivalent text explanations in the README.

Validation

• all 32 regression tests passed
• strict MkDocs build passed
• complete public-alpha built-wheel acceptance passed
• unsigned governed execution failed closed
• simulator-only trust and hardware prohibition were preserved
• independent evidence-chain verification passed
• README local links were verified
• all images have alternative text
• all SVGs include accessible titles and descriptions
• unsafe TestPyPI dependency resolution was rejected
• internal cloud identifiers are absent from the README

Visibility

This pull request does not change repository visibility or GitHub settings.
The repository remains private until the final launch-control gate.