fix: stream large query results through pager

[wuyua9]

Summary

• bound table formatting to 500 buffered rows before switching to tab-safe streaming
• stream query output through direct output or pager without building one giant string
• preserve existing small-result table formatting and add focused streaming tests

Context

Addresses #8, which reports that large query results can exhaust memory.

Validation

• go test ./...
• git diff --check