[Elements2720]

Summary

Fixes #6947.

SSR style generation now:

SSR styles
-> add sty-id for every component stylesheet
-> client hydration finds existing sheet
-> reuse it once per scope
-> HMR remains allowed to replace stylesheet text

Changes:

• Mark every SSR/HMR-created component style with sty-id, including light-DOM components.
• Preserve server stylesheet text during hydration and record the scope as applied.
• Keep existing HMR stylesheet replacement behavior.
• Add regression coverage for light-DOM SSR markers, multi-instance hydration reuse, and HMR updates.

Evidence

• Before: the two new regression tests failed: light-DOM SSR styles had no sty-id, and hydration replaced server-rendered text.
• After: focused runtime hydration/style checks passed: 3 suites, 20 tests.
• npm run build -- --ci passed.
• npm run test.dist passed.
• npm run test.testing passed.
• npm run tsc.prod passed.
• ESLint and Prettier checks passed for the changed files.

The full root Jest run was also attempted. In this environment it hit an unrelated existing failure in src/compiler/output-targets/dist-lazy/test/generate-lazy-module.spec.ts and later ran out of memory; the focused runtime suite passed independently (65 suites, 685 tests) before the final HMR-only test addition.

Merge Danger

Door: two-way

Blast Radius: runtime

The change is limited to runtime stylesheet registration/reuse and its tests. HMR remains explicitly covered, while hydration no longer mutates server stylesheet text.