chore: Add @launchdarkly/eventsource-node package scaffold

[joker23]

Summary

First layer of the eventsource migration stack. Adds the @launchdarkly/eventsource-node package scaffold: package identity (package.json, tsconfigs, tsup, jest, typedoc, docs), the dependency-free retryDelay and capacity modules with their unit tests, and the public option/event types, which derive from the @launchdarkly/js-sdk-common platform contract. src/index.ts is a scaffold that exports only the types; the EventSource implementation and its export arrive in the next layer. Also wires the package into the root workspaces and tsconfig references, and adds a build-test CI workflow with a TODO for contract tests (completed later in the stack).

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Note

Overview
Introduces @launchdarkly/eventsource-node as the first layer of in-repo EventSource work: package metadata, build/test tooling (tsup, Jest, TypeDoc), license/README/changelog, and monorepo wiring (Yarn workspace, root tsconfig project reference, dedicated eventsource-node.yml CI via shared ./actions/ci).

The public surface is intentionally minimal: src/index.ts exports only types aligned with @launchdarkly/js-sdk-common (init options, event payloads, Node TLS/proxy options); the EventSource client is deferred to a follow-up layer. This PR does land two ported helpers—retryDelay (backoff, jitter, reset interval) and capacity (buffer growth with a fixed 8192 minimum instead of reading Buffer.poolSize)—each with unit tests; Jest is pinned to maxWorkers: 1 to avoid port collisions from test helpers. Contract tests in CI are left as a TODO.

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[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 26360 bytes
Compressed size limit: 29000
Uncompressed size: 129188 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 32563 bytes
Compressed size limit: 34000
Uncompressed size: 116654 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 25568 bytes
Compressed size limit: 44000
Uncompressed size: 166115 bytes

[joker23]

should be a ts version of https://github.com/launchdarkly/js-eventsource/blob/main/lib/capacity.js

[joker23]

should be a ts version of https://github.com/launchdarkly/js-eventsource/blob/main/lib/retry-delay.js

[joker23]

NOTE: I decided to take the rejectUnauthorized option out of top level option. This should be fine from an API standpoint as I will update the call sites.

[devin-ai-integration]

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