Task: Send raw bytes instead of base64 for image-carrying NATS messages

Image byte payloads currently travel base64-encoded inside a JSON string field (via rfl::json's default std::vector<uint8_t> handling), even though the compression task's generic transport-level gzip already compresses that JSON+base64 text as opaque bytes. This task removes the base64 layer specifically for image-carrying messages, so the raw SVG bytes go straight into the (already compressed) NATS payload with no encode-then-immediately-decode round trip.

Not load-bearing: the compression task's own data shows gzip-with- base64 already gives a 2.5x margin against the worst measured case. This is a further ~25% payload reduction plus removing pure CPU/ bandwidth waste, not a bug fix. Needs a custom wire encoding for image-carrying messages specifically (bypassing rfl::json's default behaviour for that field), which is why it's split from the compression task rather than bundled with it.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Superseded on 2026-07-28 by Make NATS wire format configurable: JSON/MessagePack, decided once at startup. While scoping this task's custom binary framing for image-carrying messages specifically, it became clear the base64 overhead isn't image-specific – every rfl::json-encoded NATS message pays it for any byte-vector field. reflect-cpp already supports MessagePack (rfl::msgpack), which natively packs std::vector<uint8_t> as binary with zero custom framing code. The new story solves this task's goal generically, for every message type, via a single .env-driven format choice rather than a bespoke per-message wire format.

Manual QA scenarios (scaffolded via compass add test_scenario, run through the QA Validation Runner panel) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.