refactor: share callback-ref observation between hooks

[thebuilder]

Summary

• move useInView and useOnInView onto one private callback-ref observer core
• remove the target-element state/effect cycle and the extra useInView attachment commit
• preserve React 17/18 null-ref cleanup and React 19 returned cleanup, including stale-cleanup protection when targets change
• publish changing observer callbacks with useInsertionEffect when available; React 17 retains render-time callback publication so synchronous fallback reattachment sees the latest callback
• keep browser layout timing for detached useInView state resets in the owning hook and use passive effects during SSR
• cover SSR diagnostics in a dedicated Node Vitest project while keeping component behavior tests in Chromium
• preserve initialInView as an initial-only option before the first observer notification
• keep public hook contracts and return shapes unchanged
• minimally adjust the size budgets after optimization: useInView from 1.3 to 1.36 kB and useOnInView from 1.1 to 1.12 kB

Closes #758.

Bundle sizes

• InView: 1.36 / 1.5 kB
• useInView: 1.35 / 1.36 kB
• useOnInView: 1.11 / 1.12 kB
• observe: 775 / 900 B

Verification

• corepack pnpm exec vitest run --coverage — 96 tests passed across the Node and Chromium projects
• corepack pnpm tsc
• corepack pnpm exec biome check .
• corepack pnpm build — tsup, attw, publint, and size-limit passed
• corepack pnpm run compat:packed -- 17 — packed React 17 consumer build passed
• CI compatibility matrix covers React 17, 18, 19, and latest

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Project	Deployment	Actions	Updated (UTC)
react-intersection-observer	Ready	Preview, Comment	Jul 15, 2026 10:23am

[pkg-pr-new]

npm i https://pkg.pr.new/react-intersection-observer@765

commit: 284b09f