Fix LaTeX dollars

Investigate and fix the actual $ usages in Markdown content. The $ fall into three classes: - Currency (escape these) — $1, $2 billion, R$ 549 → these pairs cause all the warnings - Real math (leave alone) — $\rightarrow$, $O(1)\text{ streaming}$ → valid, no warnings - Shell code (leave alone) — $(curl…), [ZSH_CUSTOM], $HOME → inside code blocks Execute in 4 steps: - Investigate — greps the content, classifies every $ into currency / real math / shell code, and reports counts before changing anything. - Apply — checks the tree is clean, then writes and runs the exact tested Python script (code-fence-, inline-code-, frontmatter-, and math-span-aware; idempotent via the (?<!\\) lookbehind so re-running never double-escapes). - Verify the diff — the safety net: greps that must print nothing for real math ($\rightarrow$, \text) and shell vars ($HOME, $(…), [VAR]). If anything legit was touched, it tells you to git checkout -- . and stops. - Print instructions — outputs the build-verify and commit/push commands for user to run. Do not autonomously run any build, commit, or push.