[Typography] Quieter default text, tighter EuiTitle tracking, align EuiText size with body

[ryankeairns]

Part of #9893

Goal

Typography quick wins for the Kibana visual refresh — not the broader token track in #9894 (surfaces, radii, borders, semantic spacing).

Scope

• Paragraph / body color (theme): soften Borealis colors.textParagraph (and legacy colors.text) toward subdued — light shade110, dark shade40. Root/html and inheriting components (including bare EuiText) pick this up. Leave textSubdued and textHeading unchanged so emphasis hierarchy remains
• EuiText: keep default color as inherit
• EuiTitle: decrease letter-spacing via theme title token (font.title.letterSpacing); keep darker textHeading color
• EuiText: default size → s to match font.body.scale / unstyled body (~14px); keep default size in sync with body scale going forward

Out of scope

• Surfaces, borders, radii, semantic spacing (Token updates: surfaces, radii, borders, semantic spacing #9894)
• Tabs, buttons, panels, forms, tables (beyond inherited/textParagraph text color)
• Kibana app/panel title size/weight (product-side)
• Changing textHeading or textSubdued values

Implementation

#9900