feat: initial nonicons.nvim rewrite

Problem: the original ya2s/nvim-nonicons fork diverged enough that
maintaining it as a fork was pointless. The set_icon() approach broke
with modern nvim-web-devicons, and the extensions directory coupled
integrations that belong in user config.

Solution: from-scratch rewrite with function wrapping + table mutation
override engine, vim.g.nonicons config pattern, vendored mapping with
integer codepoints, health check, vimdoc with recipes, and no built-in
plugin integrations.

.gitignore

@@ -0,0 +1,12 @@
+doc/tags
+*.log
+
+.*cache*
+CLAUDE.md
+.claude/
+
+node_modules/
+
+result
+result-*
+.direnv/