Add streaming Writer support (html-write, page-write)

[escherize]

Summary

• Extracts a private emit-parsed! helper that centralizes parse -> validate -> emit logic
• Refactors html to delegate to emit-parsed! (behavioral no-op, all existing tests pass)
• Adds html-write and page-write that write directly to a java.io.Writer instead of building a string in memory

This enables faster time-to-first-byte and lower memory usage for large or lazily-generated pages. The approach works because all emit multimethods already write exclusively through an append! function argument — streaming support just provides a different append! that writes to a Writer.

Design decisions

• Writer not OutputStream: HTML is characters, not bytes. Callers wrap OutputStream in OutputStreamWriter (one-liner)
• Writer-first arg order: follows spit+io/copy convention
• No auto-flush: caller controls flushing; auto-flushing on every append would destroy buffered-writer performance
• No changes to existing API html and page behave identically

Closes #35

Test plan

• All 35 existing tests pass (180 assertions, 0 failures)
• New html-write-test: basic output, equivalence with html, opts passthrough, component support
• New page-write-test: doctype prefix + content