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.

README.md

@@ -0,0 +1,180 @@
+# nonicons.nvim
+
+**Nonicons for Neovim**
+
+Replace [nvim-web-devicons](https://github.com/nvim-tree/nvim-web-devicons)
+glyphs with icons from the [nonicons](https://github.com/yamatsum/nonicons)
+font. Any plugin that uses nvim-web-devicons automatically displays nonicons
+glyphs.
+
+## Requirements
+
+- [nvim-web-devicons](https://github.com/nvim-tree/nvim-web-devicons)
+- [nonicons font](https://github.com/yamatsum/nonicons/releases) installed in
+  your terminal
+
+## Installation
+
+Install the nonicons font and configure your terminal to use it as a fallback:
+
+**ghostty**
+
+```
+font-family = Nonicons
+font-family = YourMainFont
+```
+
+**kitty**
+
+```
+symbol_map U+F101-U+F219 Nonicons
+```
+
+**wezterm**
+
+```lua
+config.font = wezterm.font_with_fallback({
+  'YourMainFont',
+  'Nonicons',
+})
+```
+
+Install with your package manager of choice or via
+[luarocks](https://luarocks.org/modules/barrettruth/nonicons.nvim):
+
+```
+luarocks install nonicons.nvim
+```
+
+**lazy.nvim**
+
+```lua
+{
+  'barrettruth/nonicons.nvim',
+  dependencies = { 'nvim-tree/nvim-web-devicons' },
+}
+```
+
+The plugin applies overrides automatically. No `setup()` call is needed.
+
+## Configuration
+
+Configure via `vim.g.nonicons` before the plugin loads:
+
+```lua
+vim.g.nonicons = {
+  override = true,  -- default; set false to disable devicons wrapping
+}
+```
+
+## Usage
+
+```lua
+local get = require('nonicons').get
+
+get('lua')        -- returns the nonicons lua character
+get('python')     -- returns the nonicons python character
+get('git-branch') -- returns the nonicons git-branch character
+```
+
+The raw mapping table is available for advanced use:
+
+```lua
+local code = require('nonicons').mapping['lua']
+local icon = vim.fn.nr2char(code)
+```
+
+## Recipes
+
+**lualine** — mode icons:
+
+```lua
+local get = require('nonicons').get
+
+require('lualine').setup({
+  sections = {
+    lualine_a = {
+      {
+        'mode',
+        fmt = function(mode)
+          local map = {
+            NORMAL = get('vim-normal-mode'),
+            INSERT = get('vim-insert-mode'),
+            VISUAL = get('vim-visual-mode'),
+            REPLACE = get('vim-replace-mode'),
+            COMMAND = get('vim-command-mode'),
+          }
+          return map[mode] or mode
+        end,
+      },
+    },
+  },
+})
+```
+
+**nvim-notify**:
+
+```lua
+require('notify').setup({
+  icons = {
+    ERROR = require('nonicons').get('x-circle'),
+    WARN = require('nonicons').get('alert'),
+    INFO = require('nonicons').get('info'),
+    DEBUG = require('nonicons').get('bug'),
+    TRACE = require('nonicons').get('play'),
+  },
+})
+```
+
+**mason.nvim**:
+
+```lua
+require('mason').setup({
+  ui = {
+    icons = {
+      package_installed = require('nonicons').get('check'),
+      package_pending = require('nonicons').get('sync'),
+      package_uninstalled = require('nonicons').get('x'),
+    },
+  },
+})
+```
+
+**oil.nvim**, **fzf-lua**, **telescope.nvim** — no configuration needed. These
+plugins read from nvim-web-devicons which nonicons.nvim wraps automatically.
+
+**nvim-tree**:
+
+```lua
+local get = require('nonicons').get
+
+require('nvim-tree').setup({
+  renderer = {
+    icons = {
+      glyphs = {
+        default = get('file'),
+        symlink = get('file-symlink-file'),
+        folder = {
+          default = get('file-directory-fill'),
+          open = get('file-directory-open-fill'),
+          symlink = get('file-submodule'),
+          empty = get('file-directory-outline'),
+          arrow_open = get('chevron-down'),
+          arrow_closed = get('chevron-right'),
+        },
+      },
+    },
+  },
+})
+```
+
+## Documentation
+
+```vim
+:help nonicons.nvim
+```
+
+## Acknowledgements
+
+- [yamatsum/nonicons](https://github.com/yamatsum/nonicons) — icon font
+- [ya2s/nvim-nonicons](https://github.com/ya2s/nvim-nonicons) — original plugin