[codex] Document writer interpolation sugar

[bobzhang]

Summary

• Adds writer <+ "...\{x}" documentation near string interpolation.
• Shows the StringBuilder usage in the existing interpolation example.
• Documents that literal segments call write_string and interpolated expressions call write_object, not write.
• Notes that String values use Show formatting under write_object, so they are quoted.

Validation

• moon version -> moon 0.1.20260509 (8d0f1a5 2026-05-09)
• moon run - with StringBuilder and <+ verified hello 42 output
• moon run - with &Logger and <+ compiled successfully
• custom writer with write_string + write_object compiled successfully
• custom writer with write_string + write failed as expected: Type W has no method write_object
• git diff --check