fix(docs-site): restore the Copy page split button

[vyctorbrzezowski]

Summary

Restore the page-header actions as one split control with two independent targets: the left segment copies the page and the right segment opens the contextual menu.

Cause

The design-token migration applied var(--oc-radius-control) to all four corners of both segments. Combined with the existing 1 px overlap, that rendered two rounded pills pressed together instead of one composed control.

Fix

• keep var(--oc-radius-control) only on the primary segment outer-left corners
• keep the same token only on the menu segment outer-right corners
• retain the existing 1 px overlap so the two borders resolve into one continuous hairline
• add computed-style and geometry coverage for radii, height, seam width, wrapping, distinct hit targets, menu opening, alignment, and Escape handling across desktop, narrow, light, and dark views

No DOM, interaction behavior, menu content, or shared button styling changed.

Before / After

Current on main	Proposed in this draft PR

Theme and narrow-view proof

Proposed in this draft PR — dark	Proposed in this draft PR — narrow dark

Validation

• npm test — 4 passed
• DOCS_SITE_PREVIEW_INCLUDE_FIXTURE=1 npm run docs:build:preview — 31 pages built
• npm run docs:visual — passed against the full renderer build
• npm run docs:smoke — passed against the full renderer build
• node scripts/docs-site/r2-prepare.mjs — 84,806 objects prepared
• node --check scripts/docs-site/assets.mjs
• node --check scripts/docs-site/smoke.mjs
• node --check scripts/docs-site/visual-smoke.mjs
• git diff --check

The regression test fails on the previous styling because both segments compute four rounded corners instead of external corners only.

Scope

Only the docs renderer CSS and its smoke coverage changed. Generated docs/** content and dist/docs-site/** output are not included.

Proof head: 11a2f06c912b41de1b9a985ae618f5346aa0fdbe