Centralize CHASM scheduler lifecycle transitions

[chaptersix]

What changed?

• Add a total BufferedStart lifecycle classifier with an explicit invalid state over Attempt, RunId, Completed, BackoffTime, and the retry evaluation time.
• Co-locate the classifier and named transitions in the scheduler-private internal package, shared directly by scheduler runtime and migration code without forwarding wrappers.
• Route lifecycle mutations through those transitions for unprocessed, deferred, ready, retrying, started, and completed starts.
• Add direct classifier and transition tests plus focused chasmtest.Engine coverage for the unprocessed-to-ready and ready-to-retrying persisted transitions.
• Pin V1 migration normalization and the existing completion-before-start and backoff-equality behavior.

Production behavior is unchanged. Retry handling advances Attempt exactly once and preserves its existing backoff deadline. A late start result still fills RunId and StartTime without erasing an earlier completion, and a backoff deadline equal to the evaluation time remains ready.

Why?

This is PR 2 of the CHASM scheduler readability stack. It gives the compound BufferedStart tuple one vocabulary and one production mutation boundary without introducing the buffer planner, execution-phase split, property generators, or unrelated test migrations planned for later PRs.

Stack

• PR 0: Characterize CHASM scheduler task validity #35, main <- sch-readable
• PR 1: Separate CHASM scheduler action capacity APIs #36, sch-readable <- sch-readable-action-capacity
• PR 2 (this PR): Centralize CHASM scheduler lifecycle transitions #38, sch-readable-action-capacity <- sch-readable-lifecycle

How did you test it?

• built
• run locally and tested manually
• covered by existing tests
• added new unit test(s)
• added new functional test(s)

go test -tags test_dep ./chasm/lib/scheduler/internal -run 'TestClassifyBufferedStart|TestBufferedStartTransitions' -count=1
go test -tags test_dep ./chasm/lib/scheduler -run 'TestBufferedStartLifecycleTransitionsEngine|TestExecuteTask_Validate_BackoffEqualToLPTIsEligible|TestExecuteTask_EngineCharacterizesCurrentWorkValidity|TestHandleNexusCompletion_ReenablesDeferredStarts|TestHandleNexusCompletion_CompletionBeforeStart' -count=1
go test -tags test_dep ./chasm/lib/scheduler/migration -run 'TestLegacyToCreateFromMigrationStateRequest|TestConvertRunningWorkflowsToBufferedStarts_UniqueRequestIDs' -count=1
go test -tags test_dep ./chasm/lib/scheduler/... -count=1
GOLANGCI_LINT_BASE_REV=sch-readable-action-capacity GOLANGCI_LINT_FIX=false make lint-code
git diff --check

Potential risks

The main risk is changing a compound lifecycle tuple while centralizing its writes. Direct classifier tests cover every named state, invalid tuples, and retry-time boundaries; engine and existing race tests protect persisted transition behavior. The migration assertions pin pending normalization and running/completed tuple construction.