fix(react-headless-components-preview): share anchor-name across positioned popovers

[micahgodbolt]

Fixes

Part of #36346.

What

usePositioning wrote a unique anchor-name onto the trigger element and overwrote anything already there:

effectiveTarget.style.setProperty('anchor-name', anchorName); // clobbers

When two positioned popovers share one trigger — e.g. a hover Tooltip + a click Menu on the same button — the two usePositioning instances clobber each other's anchor-name. anchor-name is single-valued here, so the last writer wins; the other popover's position-anchor points at a name that no longer exists and falls back to default (static / top‑left) placement.

Rendered proof (menu closed, tooltip visible)

button   anchor-name   = "--popover-anchor-r1"          ← only the MENU's name survives
div[role=tooltip popover=hint]      position-anchor = "--popover-anchor-r2"   ← orphaned
div[role=presentation popover=auto] position-anchor = "--popover-anchor-r1"   ← menu, OK

How

anchor-name accepts a comma-separated list. Each usePositioning instance now appends its own name and removes only its own on cleanup, preserving any author-set or sibling anchors:

const names = readAnchorNames();
if (!names.includes(anchorName)) {
  effectiveTarget.style.setProperty('anchor-name', [...names, anchorName].join(', '));
}
// cleanup removes only this instance's name

Single-popover triggers are unchanged; N popovers per trigger now each resolve their own position-anchor.

Tests

Added two unit tests to usePositioning.test.tsx:

• two instances on one trigger → anchor-name contains both distinct names
• a pre-existing author-set anchor-name is preserved

All 19 usePositioning tests pass; Tooltip suite (16) unaffected.

[github-actions]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
react-headless-components-preview react-headless-components-preview: entire library	248.544 kB 71.466 kB	248.779 kB 71.541 kB	235 B 75 B

_{🤖 This report was generated against 19d82d85bda458b0af3fa4e0814bf3232fc78594}

[github-actions]

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