Prepare admission tickets atomically

[arasovic]

Summary

• add one concrete atomic Gate.Prepare batch enqueue operation
• validate a same-run unique-worker batch before IDs or durable state work
• generate every ticket ID before one locked state transition
• append consecutive queued tickets under the supervisor-owned Gate identity without leasing capacity
• preserve Enqueue and foreground behavior while retaining existing stale-owner reconciliation semantics

Verification

• go test -race ./internal/admission -run TestPrepare -count=20
• go test -race ./internal/admission -count=5
• concurrent Prepare/Enqueue FIFO race repeated 20 times
• random-ID, collision, overflow, corrupt/symlink state, save failure, and stale-reaping regressions
• mutation proofs for whole-batch validation, exact one-save atomicity, duplicate-ID precheck, and queued-before-Wait state
• npm run verify
• go build ./...
• Windows compile-only admission package test
• independent Pi Worker Standards and Spec/simplicity reviews: no actionable findings

Closes #195

Parent: #177