Eliminate keystroke render lag from redundant history wrapping

[shellicar]

Summary

• Keystroke renders no longer rewrap unchanged history lines
• Incremental cache wraps only new entries since last sync
• Cache invalidates on terminal resize

Closes #166

[bananabot9000]

LGTM. Incremental wrapping cache is clean -- append-only displayBuffer + column-change invalidation is the right cache strategy. Perf tests enforce the sub-1ms contract. Functional tests cover the important cases (visibility, wrapping, resize invalidation).

The screen?: Screen injection point works for now. Can revisit if the constructor grows further.

68% context token reduction from the skill sync script + render lag eliminated in the same night. Productive session. 🍌