Encrypt private ledger data directly into serialised entries

[eddyashton]

Background

A perf profile of the 3,000,000-request pi_basic workload attributes approximately 24.06% inclusive CPU to CommittableTx::serialise(). Across the full profile, memory copy/set leaves account for 5.06% and allocation/refcount leaves for 16.25%; within transaction serialization alone these categories account for approximately 1.08% and 2.02% of total CPU respectively.

The current private-domain serialization path creates and copies through several buffers:

1. GenericSerialiseWrapper builds separate public and private writer buffers.
2. serialise_domains() allocates the final ledger entry.
3. It allocates a serialised_hdr vector and an encrypted_private_domain vector.
4. KeyAesGcm_OpenSSL::encrypt() allocates another local ciphertext vector, fills it, and move-assigns it over the caller-provided vector.
5. The GCM header, public domain, and encrypted private domain are copied into the final ledger entry.

This work is separate from reusing initialised AES-GCM contexts (#8168) and should be implemented/measured independently.

Proposed implementation

• Add a span/caller-owned-output encryption API alongside or beneath the existing vector API.
• Precompute the complete serialised-entry layout and allocate the final ledger entry once.
• Write the entry header, public-domain size, and public-domain bytes directly to their final positions.
• Encrypt the private domain directly into its final ciphertext span in the ledger entry, while continuing to authenticate the public domain as AAD.
• Serialize the fixed-size GCM header/tag directly into the reserved header region once encryption completes.
• Remove the local ciphertext allocation in KeyAesGcm_OpenSSL::encrypt() when the caller already provides correctly sized output storage.
• Consider moving/taking writer buffers where direct output is not possible, but keep that secondary to the single-allocation final layout.

Compatibility and correctness coverage

• Preserve the ledger entry format byte-for-byte; this must not require a compatibility-version change.
• Verify public-only, private-only, mixed-domain, and empty-private-domain transactions.
• Verify snapshots and historical-secret serialization/decryption.
• Verify ledger replay and recovery against entries produced before and after the change.
• Ensure encryption failure does not expose or replicate a partially constructed entry.
• Preserve the existing rule that public-domain bytes are GCM additional authenticated data.
• Retain vector-based public APIs where required, implemented via the new output-span primitive if practical.

Measurement

Add focused crypto/serialization benchmarks for small transaction payloads and compare allocation counts, bytes copied, and throughput. Run an A/B pi_basic benchmark/profile independently from #8168 so context-reuse and reduced-copy gains remain attributable.