Add service isolation (ctx.isolate) and intercept (ctx.intercept)

[ryudoawaru]

Summary

Round 3 of aligning with upstream packages/core (4.0.0-rc.8): service isolation and per-caller config interception.

• ctx.isolate(name, label = nil) — returns a ctx view whose service name lives in its own realm. The service store is now keyed by isolate key: the name itself in the default realm, a unique token (or a user-supplied label) in isolated ones. Passing the same label to two isolate calls shares one realm between them.
• Realm-aware notify — Context#notify takes { name => key } pairs and only touches fibers whose ctx resolves the name to the same key, so provides, provider swaps, and teardowns stay realm-local. Fiber#provided tracks { name => key } for ACTIVE-boundary notifications.
• ctx.intercept(name, config) — carries per-caller config on the ctx view. Implemented as copy-on-write with the chain pre-merged at intercept time (upstream walks the prototype chain in Service[resolveConfig] instead). Hash-form inject configs (inject: { foo: {...} }) become intercepts on the plugin ctx, mirroring upstream fiber.ts:139. Consumers read the effective config with ctx.resolve_config(name, base).
• One deliberate divergence (stricter than upstream): the service access walk checks the realm key of provided services at the terminal fiber. Upstream's walk is name-keyed at that point, which would let a plugin under an isolated view leak the default-realm service once the walk reaches root.

Test plan

• bundle exec rspec — 53 examples, 0 failures (new spec/cordis/isolate_spec.rb mirrors upstream tests/isolate.spec.ts: isolated context, shared label; plus duplicate-per-realm, boundary walk, intercept resolution, and inject-config conversion)
• bundle exec ruby examples/demo.rb still runs clean

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