docs: add vertical spacing guidance from MWPW-201396

[qiyundai]

Summary

• Records the Design/Consonant/Authoring decision from MWPW-201396: vertical block spacing is a Figma/authoring concern, not something to hard-code in engineering implementations.
• Adds a ### Vertical Spacing subsection to CLAUDE.md under Coding Standards, so future block work (and Claude Code) follows this convention: no hard-coded top/bottom padding/margin, authors keep flexibility, and any genuinely-required baked-in spacing must be documented (value + viewport + reason).
• The specific blocks called out in the ticket (plans-hero, hub-hero, offer-hero, logo-ticker, pdf-space, brand-concierge, comparison-table-c2) are Consonant/Milo C2 blocks that don't live in this repo, so this PR ports the principle rather than a block-specific list.

Test plan

• Docs-only change — no code touched, npm test/npm run lint not applicable
• Reviewer confirms the guidance text accurately reflects the ticket's confirmed decisions

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Commits

• dfc2bab ✅ (latest)