Why: cache tokens were parsed but never accumulated, the usage event
carried only cumulative totals and was skipped on terminal paths
(audit N20 residual), and stream retry was host-opt-in so subagents,
the verifier, and squad runs were never retried.
Evidence: GOWORK=off go vet + go test -count=1 ./... green;
TestUsageEventSplitsLastAndCache (event carries Last*/cache),
TestUsageAccumulatesCacheTokens, TestBudgetStopEmitsTerminalUsage,
TestDefaultRetryAttemptsTwo (0 value = 2 retries with
auto_retry_start events), TestNegativeRetryAttemptsDisables (negative
= off), TestRetryNeverDuplicatesForwardedDeltas (the only-before-first-delta
invariant holds under the library default).
Impact: usage events now carry Last* and cache fields and are emitted
on budget/error terminal paths too; Config.RetryAttempts defaults to
2 (negative disables) so every host gets the existing retry semantics
without wrapping; default (unset) host wrappers should be retired to
avoid double retries.
Refs: Agent-System-Documents#43 (N3 plan WP6), audit N20
weichen