computeNonMessageTokens / computeNonMessageBreakdown re-tokenize the system
prompt and every tool's wire schema (per-tool JSON.stringify) on each call,
but the per-turn compaction and context-threshold paths call them several
times (getContextBreakdown twice, #estimateStoredContextTokens once) over
inputs that change at most once per turn. Memoize on the identity of
(systemPrompt, tools, skills) -- the same stable refs the StatusLineComponent
cache already trusts -- so the expensive parts run at most once per input
change instead of per call.