Add XHS topic generation timing

[walker1211]

Summary

• Split XHS auto-publish option building into detailed timing stages.
• Add a dedicated generate_topics timing line to measure AI topic tag generation separately from browser topic input.
• Cover the new timing output with a regression test.

Test plan

• go test ./cmd/mark2note -run 'TestBuildAutoPublishXHSOptionsReportsTopicGenerationTiming|TestRunAutoPublishXHS' -count=1
• go test ./...
• go build -o ./mark2note ./cmd/mark2note