chore: Scaffold @launchdarkly/eventsource with transport-independent core modules

[joker23]

Summary

This PR scaffolds @launchdarkly/eventsource, a new shared package that will hold a fetch-based EventSource (server-sent events) client for the LaunchDarkly SDKs. This first slice contains the package skeleton and the transport-independent core modules only; the client itself arrives in the next PR in the stack.

Package scaffold:

• packages/shared/eventsource at version 0.0.1, with dual ESM/CJS output via tsup, jest, oxlint/oxfmt, and typedoc.
• CI workflow, release-please wiring, and docs-publish registration.

Core modules (no client or transport logic yet):

• httpHelpers.ts: structural types for the exact WHATWG fetch subset the client uses (FetchFn, FetchResponse, and friends). Any fetch-shaped transport can satisfy them, and the published declarations resolve without the DOM lib.
• Event.ts / types.ts: the event payload types (MessageEvent, ErrorEvent, OpenEvent, RetryingEvent, ClosedEvent), the typed EventSourceEventMap, and the pluggable EventListenerRegistry contract. The payload shapes stay interchangeable with the platform types in @launchdarkly/js-sdk-common; the typeCompatibility suite (added with the client) enforces this.
• listenerRegistry.ts: the default Map-backed registry. Its add/remove/dispatch semantics match Node's EventEmitter, so an emitter-backed substitute stays interchangeable, except that dispatch of a listenerless error type never throws.
• retryDelay.ts: the backoff/jitter retry-delay strategy, with a reset interval for connections that stay healthy.
• parsing.ts / capacity.ts: SSE byte constants, BOM handling, reconnect header-value validation, and buffer growth calculation.

Unit tests cover the registry, retry delay, and capacity modules.

---

Note

Overview
Introduces @launchdarkly/eventsource (packages/shared/eventsource) as a new shared workspace package (v0.0.1) with build/test/lint tooling, dual ESM/CJS output, TypeDoc, license/changelog, and a dedicated GitHub Actions workflow. The root package.json workspace list and tsconfig.json project reference are updated to include the package.

This slice is scaffolding plus transport-independent building blocks for a future fetch-based SSE client—the public index still notes that the full EventSource implementation is coming later. Added modules include fetch-shaped transport types (httpHelpers), W3C-aligned event payloads and EventSourceInitDict (Event, types), a default EventListenerRegistry with EventEmitter-like semantics, reconnect retry delay (backoff/jitter/reset), SSE parsing constants and header validation (parsing), and stream buffer capacity growth (capacity).

Unit tests cover the registry, retry strategy, and capacity logic; Jest is configured with maxWorkers: 1 to avoid port collisions from test helpers.

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[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 27065 bytes
Compressed size limit: 29000
Uncompressed size: 131676 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 25437 bytes
Compressed size limit: 44000
Uncompressed size: 165420 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 32573 bytes
Compressed size limit: 34000
Uncompressed size: 116865 bytes

[devin-ai-integration]

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