v2.0.0-rc1

Release candidate 1 for v2.0.0. Bundles:
- chore: VictoriaLogs event fixtures corpus (D-25-2)
- feat!: multi-source VictoriaLogs core (#34 part 1)
- feat!: multi-source observability and guardrails (#34 part 2)

Breaking changes (full text in CHANGELOG.md):
- victorialogs becomes a map of named sources
- vl_source label added to per-rule Prometheus counters
- valerter_victorialogs_up replaced by valerter_vl_source_up
- defaults.max_streams cap (default 50)
- Default throttle key now {rule}-{source}:global
- Source names restricted to ^[a-zA-Z0-9_]+$
- Mattermost footer + default webhook payload extended with source

Awaiting reporter (#34) validation against real infrastructure before
tagging v2.0.0 final.