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.

lua/nonicons/health.lua

@@ -0,0 +1,29 @@
+local M = {}
+
+function M.check()
+  vim.health.start('nonicons.nvim')
+
+  local ok, devicons = pcall(require, 'nvim-web-devicons')
+  if ok and devicons then
+    vim.health.ok('nvim-web-devicons available')
+  else
+    vim.health.error('nvim-web-devicons not found')
+  end
+
+  local result = vim.fn.system({ 'fc-list', ':family=nonicons' })
+  if result ~= '' then
+    vim.health.ok('nonicons font installed')
+  else
+    vim.health.warn('nonicons font not detected (fc-list :family=nonicons returned empty)')
+  end
+
+  local mapping = require('nonicons.mapping')
+  local count = vim.tbl_count(mapping)
+  if count > 0 then
+    vim.health.ok('mapping loaded (' .. count .. ' icons)')
+  else
+    vim.health.error('mapping table is empty')
+  end
+end
+
+return M