Add declarative partial updates HTML demo

[wyf027]

Summary

• Add a single-file Declarative Partial Updates demo under project/declarative-partial-updates-demo/index.html.
• Demonstrate native document patching with <?start>, <?marker>, and <template for="...">.
• Use streamAppendHTMLUnsafe() only to write delayed local HTML chunks; the demo does not include a DOM patching polyfill.

Validation

• Confirmed the remote compare contains exactly one file: project/declarative-partial-updates-demo/index.html.
• Checked the demo file for absence of manual patching APIs such as MutationObserver, querySelector, innerHTML, and fetch().

Notes

Supersedes closed PR #962, which was opened from an existing feature branch that included unrelated commits.