docs: lead with the presentation challenge

[aronchick]

What changed

Repositions Gloss consistently across the README and landing page:

• Gloss is the deliberately difficult presentation deck that AI struggles to render and build as a real, editable PowerPoint.
• Measurement is a supporting capability: open tools make failure modes, evidence, and comparative results easy to inspect and reproduce.
• Search, social, and structured descriptions now carry the same hierarchy.

Verification

• node launch/verify-launch.mjs passed.
• Desktop (1440px) and mobile (390px) browser renders inspected.
• Impeccable detector reports only the pre-existing intentional X-ray canvas grid advisory.

[cloudflare-workers-and-pages]

Deploying gloss with    Cloudflare Pages

Latest commit:	3c3c7a4
Status:	✅  Deploy successful!
Preview URL:	https://713cf79b.gloss-h5f.pages.dev
Branch Preview URL:	https://codex-presentation-first-pos.gloss-h5f.pages.dev

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