No mixed tab + space indentation

Block lines whose leading whitespace contains both tabs and spaces. The two render identically in many editors but bind differently to whatever consumes the file:

• Python raises TabError: inconsistent use of tabs and spaces
• make requires literal-tab recipe lines and silently drops rules whose indent contains a space prefix
• Go gofmt rewrites the indent, producing huge unrelated diffs
• ESLint / Prettier disagree about which to keep, causing CI flap

Fix#

Pick one (project-wide). Run gofmt, black, prettier --tab-width, or expand/unexpand to normalize.

For Makefiles specifically: recipe lines must start with a literal tab - but the first character must be a tab, not a space followed by a tab.