feat(ftp): add FTP/FTPS adapter via curl

Problem: canola has no way to browse or edit files on FTP servers,
despite the adapter system being designed for exactly this pattern.
curl speaks FTP natively, including FTPS (FTP over TLS), and requires
no new dependencies.

Solution: implement `lua/oil/adapters/ftp.lua` with `oil-ftp://` and
`oil-ftps://` schemes. Parses Unix and IIS LIST output, supports
`size`, `mtime`, and `permissions` columns, and implements the full
adapter API (list, read_file, write_file, render_action, perform_action).
Same-host renames use RNFR/RNTO; cross-host and local↔FTP copies use
curl download/upload through a tmpfile. Adds `extra_curl_args` config
option and documents the adapter in `doc/oil.txt`.

Based on: stevearc/oil.nvim#210

doc/oil.txt

@@ -173,6 +173,8 @@ The full list of options with their defaults:
       extra_scp_args = {},
       -- Extra arguments to pass to aws s3 when creating/deleting/moving/copying files using aws s3
       extra_s3_args = {},
+      -- Extra arguments to pass to curl for FTP operations
+      extra_curl_args = {},
       -- EXPERIMENTAL support for performing file operations with git
       git = {
         -- Return true to automatically git add/mv/rm files
@@ -993,6 +995,50 @@ S3                                                                *oil-adapter-s
     Older versions of Neovim (0.11 and earlier) don't support numbers in the
     URL scheme, so use `oil-sss` instead of `oil-s3`.
 
+FTP                                                              *oil-adapter-ftp*
+
+    Browse files over FTP or FTPS (FTP over TLS). Open a buffer with: >
+        nvim oil-ftp://[username[:password]@]hostname[:port]/[path]/
+        nvim oil-ftps://[username[:password]@]hostname[:port]/[path]/
+<
+    The `oil-ftps://` scheme connects with `--ssl-reqd`, requiring a valid TLS
+    certificate. Use `oil-ftp://` for plain FTP.
+
+    Authentication ~
+
+    Credentials can be supplied in the URL (`user:pass@host`) or stored in
+    `~/.netrc` (recommended — keeps passwords out of shell history): >
+        machine ftp.example.com login myuser password mypass
+<
+    How it works ~
+
+    The FTP adapter uses `curl` to perform all operations. Directory listings
+    come from FTP LIST output (Unix and IIS/Windows formats are both supported).
+    File reads download to a local tempfile; file writes upload from a tempfile.
+    Renames on the same server use the FTP RNFR/RNTO commands. File copies
+    between servers go through a local tempfile.
+
+    Limitations ~
+
+    Symbolic links cannot be created over FTP. Directory copies are not
+    supported (copy individual files instead). Moving or copying directories
+    across different FTP hosts is not supported.
+
+    Configuration ~
+
+    Pass extra flags to `curl` with `extra_curl_args`: >lua
+        require("oil").setup({
+          extra_curl_args = { "--insecure" },
+        })
+<
+    The adapter supports the `size`, `mtime`, and `permissions` columns.
+    Permission changes use the FTP `SITE CHMOD` command; not all servers
+    support it.
+
+    Dependencies ~
+
+    Requires `curl` (standard on Linux and macOS).
+
 Trash                                                        *oil-adapter-trash*
 
     See |oil-trash| for details on the built-in trash adapter.