fix(remote): make raw existence probes proxy-safe

[mayneyao]

Summary

• isolate HTTP raw existence probes in a dedicated connection pool
• fall back to a one-byte Range GET only for explicit HEAD 405/501 responses
• preserve auth, missing, server, and transport errors; handle empty and large segments safely
• prepare @eidos.space/graft 0.3.2

Validation

• cargo nextest run -p graft -p graft-sdk -p graft-sdk-node (236 passed)
• pnpm --dir packages/graft-sdk test (14 passed)
• real Eidos Lite staging push and clone (2 passed)
• staging Worker deployment dd911914-1a72-44cd-a4ad-b73a0fd51dc0