Stoplight 6.0 release

[bolshakov]

What 6.0 includes

• Registered, cached lights: Stoplight() now registers/reuses a light and Stoplight.light(name) is lookup-only.
• Named systems: Stoplight.register_system provides isolated configuration/infrastructure per system.
• In-process telemetry: Stoplight.telemetry / system.telemetry, with run, breach, recovery, lock, registration, and transition events. Not yet ready, we may not include this into release notes.
• Per-run error matcher overrides: light.run(tracked_errors: …), the same for skipped_errors
• Admin improvements: multi-system dashboard, persisted Redis light registry with light configuration, read-only mode, id-based (instead of name-based) routes, bundled assets.
• Redis storage redesign: decomposed stores and a new stoplight:v6 keyspace.
• Fixes: Memory windows now use monotonic time; clock-skew handling around yellow-state computation, clearer RedLight message.
• Performance improvements - needs measurement.

Breaking changes that must be added to migration guide (UPGRADING.md):

• Stoplight("payments", …) calls must have identical configuration or raise ConfigurationError.
• Per-light infrastructure configuration is no longer allowed. Affected options are data_store, notifiers, and error_notifier -- must move to Stoplight.configure
• Light cloning/builder API was removed: Light#with and the whole with_* family
• Error matchers must be named Module/Class constants, not anonymous procs or class instances.
• Redis state is reset after switching to a new key schema. No cleanup needed for old schema.
• Clock skew notification removed Use Redis Time in State & Metrics to prevent time skew problem #921

Required documentation updates:

• Update the README’s “currently browsing 5.x” banner.
• Add a Systems guide: default system vs. named systems, boot-time registration, caching, and multi-system Admin configuration. Probably not everything should go to readme, but definitely should be documented in the code.
• Add Admin documentation for read-only mode.
• Document multi-system configuration for admin
• Regenerate admin screenshot

Other tasks:

• Benchmark performance and memory usge
• Update the list of supported versions of dependencies and ruby
• Release notes
• Bump version, update supported versions in readme, security, etc.