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

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 (#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

https://github.com/elastic/eui/pull/9900