fix: use explicit id to merge meta head tags

[shamu45678]

Description

Use an explicit id as the merge key for meta head entries, regardless of attribute order, while preserving the existing first-attribute fallback for entries without an id.

This follows the maintainer's latest guidance in #5362. Unlike the closed #5363 approach, it does not special-case content or name. It also documents the behavior and adds focused tests for replacement by a shared ID and preservation by unique IDs.

Validation:

• corepack pnpm lint-staged
• corepack pnpm exec vitest run -r __tests__/unit shared/shared.test.ts (1 file, 2 tests passed)
• corepack pnpm exec tsc -p __tests__/unit --noEmit

Linked Issues

fixes #5362

Additional Context

Entries without an id retain the current first-attribute deduplication behavior, so this is backward-compatible for existing configurations.