Extract the pure SSE-payload -> collected-stream parser (AntigravityCollectedStream,
stripZeroWidth, parseAntigravityTextualToolCall, addAntigravityTextualToolCall,
processAntigravitySSEPayload/Text, flushAntigravitySSEText) verbatim into the leaf
antigravity/sseCollect.ts. Host imports the helpers it uses and re-exports
processAntigravitySSEPayload for external importers (tests).
Host 1812 -> 1671 LOC. Byte-identical bodies (verbatim multiset 135/135), leaf does
not import the host (no cycle). Credit/quota state, auth, and HTTP dispatch untouched.
Adds a split-guard; consumer tests stay green (executor-agy 8, executor-antigravity 26,
antigravity-sse-collect-socket-release, copilot-agent-antigravity-parity 6).
