feat(logging): structured JSON console logs via RING_LOG_FORMAT

[Shine-neko]

What

Adds RING_LOG_FORMAT=json to emit the console logs as structured JSON (one object per line) instead of the default human-readable text. For log shippers and structured ingestion pipelines.

This is independent of OTLP log export ([server.telemetry.logs]), which can run alongside either console format.

Example

RING_LOG_FORMAT=json ring server start | jq 'select(.level == "ERROR")'

{"timestamp":"2026-07-07T12:06:36Z","level":"INFO","fields":{"message":"Starting server..."},"target":"ring::commands::server"}

Unset (or any value other than json) keeps the existing text format, so nothing changes by default.

Changes

• main.rs: console_fmt_layer() picks the JSON or text fmt layer from RING_LOG_FORMAT; json feature enabled on tracing-subscriber
• New e2e tests/e2e/server/t19_json_logging.sh: boots the server in JSON mode and asserts the log lines parse as JSON with the expected fields
• Docs: environment-variables.md (new row), observe-and-debug.md (usage with jq)

Test

./tests/e2e/server/t19_json_logging.sh   # PASS