Make chat spacer follow the latest visible user message

[hamzamerzic]

Summary

• reserve exact reply room only while the latest user message is visible
• keep remaining room after a short reply finishes, consume it as output expands, and restore it as output collapses
• keep spacer height under one layout owner so QA, tool, and image reflows cannot strand a second reservation

Why

The spacer previously had two independent writers and a lifetime that was too broad.

The corrected rule is based on rendered ownership: the latest visible user row receives the exact remaining deficit regardless of scroll mode or turn completion. Older or off-screen user rows receive none.

Related work addressed narrower symptoms: #26 corrected spacer sizing, #83 guarded disclosure collapse, and #143 protected restoration from saved reserved space. This changes the shared ownership contract underneath those cases and removes disclosure handling as a second spacer writer.

Validation

• npm test (1,822 library tests + 47 hook tests)
• npm run build
• rendered QA/tool check: collapsed output reserved 243px, enough expanded output reduced it to 0px, and collapsing restored exactly 243px
• rendered reader check: moving the latest user message off-screen reduced reservation to 0px