docs: replace bronze-only positioning with full-pipeline layer model

[easel]

What changed

The microsite, user guides, and HELIX governance docs described tablespec as stopping at ingested bronze, with silver and gold work "downstream" and outside the product. That was never the product intent. tablespec compiles UMF specs for every layer; it covers ingested bronze most completely only because a source contract can be finished without business judgment.

This PR rewrites that positioning everywhere it appeared (21 files, docs and site only, no library code):

• Homepage – headline is now One UMF. Every runtime artifact. (an already-approved headline pattern). Kicker, lede, step three, the comparison-table intro, and the closing CTA no longer place silver outside the product. The hero blueprint marks both the ingested and silver nodes as compiled contracts via a shared .ts-node-contract class.
• Concept page – concepts/raw-ingested-silver opens with the layer model instead of "silver begins after the source-table contract". The silver section states where those decisions are declared (their own UMF spec) and what they compile to (gold SQL plans, dbt models, Lakeflow datasets).
• Metadata – FAQ JSON-LD question "Where does silver begin?" becomes "Does tablespec stop at bronze?" with the answer no. Site description, page description, and og:title updated.
• Governance – brand-voice principle 3 is now "Layers over buzzwords" with an explicit rule against scoping copy to one layer; the old phrase is added to Say This, Not That. Product vision, FEAT-026, FEAT-030, DESIGN.md, VOICE.md quick spine, and US-038 (story + AC5) rewritten to match.
• Guides – happy-path, bootstrap, worked example, and validation scope no longer say silver work "remains downstream".
• Tests – microsite.spec.ts asserts the new headline and the shared contract selector, and adds a step that fails if the "begins after the source-table contract" wording returns. Homepage desktop/mobile snapshots re-baselined on Linux.

Why

Surfaced while reviewing a DDD integration proposal: the bronze-only claims would have been cited to reject cross-source features (glossary, context map, conformance) that are in scope. The separation-of-concerns idea in the old copy is kept: silver decisions are declared in their own specs and compiled by the same step, never folded into ingestion.

Reviewer notes

• ADR-007 and ADR-014 still mention the bronze boundary. They are dated decision records and were left as-is.
• The PRD summary still calls the ingested contract "bronze completion" and definition of done for ingested bronze stays in the approved-language list. Both remain true as milestone claims, not product-scope claims.
• Verification: tests/docs 36 passed; Playwright content suite 15 passed; link-check suite 11 passed.

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