fix: terminal stability (#109)

* chore: open roadmap tracking branch for terminal continuity issues

* feat(terminal): instrument continuity lifecycle

* fix(terminal): harden transcript replay semantics

* fix(terminal): relax retention truncation

* test(terminal): add continuity regression coverage

* perf(terminal): reduce fit and resize churn via dimension-gated performFit

* fix(terminal): strengthen WebGL recovery determinism and add instrumentation

* feat(terminal): add safe renderer preference toggle (auto/webgl/canvas) on mainline

* test(terminal): add xterm 5.5 baseline performance benchmarks

* docs(guardrail): codify xterm 6.0 branch exclusion from production rollout

* Fix terminal continuity follow-up findings

* Fix benchmark and restore follow-ups

* Fix benchmark churn and typecheck issues

* Fix PR validation test and lint issues

* Stabilize updater fallback test