Ingest batch

Initial bound: 100 entries (413 batch_too_large), envelope 10 MiB (413 request_too_large). Validate envelope structure/size first, then each entry independently against IngestCommand, including wrong types or missing fields. Each raw entry is also held to the 1 MiB single-request bound (entry error entry_too_large), so the same entry bytes are never refused for size alone. HTTP 200 carries Receipt or Error per entry, with the same error codes as single submission. Same ingestion family/key as single submission. Each entry has a fresh 5-second acceptance budget, bounded by an 8-second deadline for the whole batch, including envelope decoding and validation. Caller cancellation or an earlier caller deadline also applies. An entry that times out, and every remaining entry after the batch deadline, returns retryable content_unavailable. Retry with the same entry keys to recover the original Receipts. If the envelope cannot be read before the deadline, HTTP 503 returns retryable content_unavailable without attempting entries. Reuse entry keys after unknown transport outcomes.