Fix blank lines accumulating between keymaps array and postamble

[yoichiro]

Summary

• Fixed a bug where each Visual Editor round-trip (parse → generate) added an extra blank line between the }; closing the keymaps array and the postamble code (e.g.,
  layer_state_set_user)
• Root cause: generateKeymapC() appended };\n before the postamble, but the postamble already starts with \n from the parser — resulting in a duplicated newline
  on every cycle
• Added a round-trip regression test that verifies blank lines stay stable across 5 consecutive parse/generate cycles

Test plan

• All 154 workbench tests pass
• Open a keymap.c with postamble code in the Workbench
• Edit a key in the Visual Editor, switch to Code Editor — verify no extra blank lines appear
• Repeat several times — verify blank lines do not accumulate

🤖 Generated with Claude Code