Add XHS publishing stage timing logs

[walker1211]

Summary

• Add stable MARK2NOTE_TIMING stderr lines for generation, rendering, and XHS publishing stages.
• Instrument key XHS browser publishing steps so slow uploads, topic input, submit, and confirmation phases are visible.
• Keep help output clean while retaining default timing output for real generation and publish paths.

Test plan

• go test ./...
• go build -o ./mark2note ./cmd/mark2note
• ./mark2note --help smoke test with empty stderr