fix: make observe cleanup idempotent

[thebuilder]

Summary

• make every cleanup returned by observe() safe to call repeatedly
• prevent repeated cleanup from removing another callback sharing the same observer
• cover shared-subscriber delivery and final observer teardown with a regression test

Fixes #757.

Verification

• corepack pnpm exec vitest run src/__tests__/observe.test.ts
• corepack pnpm exec vitest run
• corepack pnpm exec tsc --noEmit
• corepack pnpm exec biome check src/observe.ts src/__tests__/observe.test.ts
• corepack pnpm build

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[vercel]

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Project	Deployment	Actions	Updated (UTC)
react-intersection-observer	Ready	Preview, Comment	Jul 14, 2026 11:13am

[pkg-pr-new]

npm i https://pkg.pr.new/react-intersection-observer@763

commit: 6ff366b