Resolve #2755: Settle Studio sidecar on partial-body close

[ayden94]

Summary

Studio sidecar ingestion could hang indefinitely when a local client closed the socket after sending only a partial request body. The readBody reader only listened for end and error events, so a partial-body close (where neither event fires) left the ingestion Promise pending forever, allowing a malformed local client to consume sidecar work without bound.

Linked context: Closes #2755

Changes

• packages/cli/src/studio/sidecar.ts: readBody now binds the request close event to body-reader cancellation. A settled guard ensures only the first of end/error/close wins, and all listeners are removed once settled. close rejects with a descriptive bounded error distinguishing zero-body vs partial-body close.
• packages/cli/src/studio/sidecar.ts: writeJson now guards against writing to an already-ended response, so the bounded error completion path does not throw when the client socket is already closed.
• packages/cli/src/studio/sidecar.test.ts: Added two regression tests — one for partial-body close (client sends partial JSON then closes) and one for zero-body close (client sends headers only then closes). Both assert the sidecar does not accept the event and remains usable.
• packages/cli/README.md / packages/cli/README.ko.md: Documented the partial-body close settlement invariant in the Studio sidecar security defaults section (EN/KO parity).
• .changeset/studio-sidecar-partial-body-close.md: Patch changeset for @fluojs/cli (CLI-launched sidecar lifecycle is documented tooling behavior).

Testing

• pnpm --filter @fluojs/cli test — 393 tests passed (13 files), including the 2 new partial-body close regression tests.
• pnpm --filter @fluojs/cli typecheck — passed.
• pnpm build — full monorepo build passed.

Release impact

• This PR has consumer-visible release impact and includes a changeset.

The sidecar ingestion lifecycle is documented @fluojs/cli tooling behavior. A malformed local client could previously hang sidecar work indefinitely; this change settles ingestion with a bounded error completion. Patch bump is appropriate because no public API surface or documented ordering changes — only a latent hang is fixed.

Public export documentation

• Changed public exports include a source-level summary.

No public exports changed. The readBody function is module-internal; the StudioSidecar public interface is unchanged.

Behavioral contract

• No documented behavioral contracts were removed without migration notes.
• New behavioral contracts are documented in the affected package README.
• Intentional limitations are explicitly stated (not silently removed).
• Runtime invariants are covered by regression tests.

The new invariant — "the sidecar settles ingestion requests with a bounded error completion when a local client closes the socket after sending only a partial request body" — is documented in packages/cli/README.md and packages/cli/README.ko.md, and covered by regression tests in packages/cli/src/studio/sidecar.test.ts.

Platform consistency governance (SSOT)

• SSOT English/Korean mirror structure remains synchronized for changed governance docs.

The CLI README EN/KO pair was updated in lockstep. No platform contract docs or package-surface docs changed.