fix(examples): make 04-files runnable across cpp/rust/elixir/lua/php/ruby/zig

[Nic-dorman]

Summary

The `04-files` examples in 7 SDKs were shaped as documentation snippets — they hardcoded `/tmp/example.txt` / `/path/to/myfile.txt` placeholder paths that don't exist on disk. Against a live daemon, the daemon returned `HTTP 400: Bad request: invalid path` from the first `file_cost` call and the example exited 1 (php: 255, same root cause). Same shape as the antd-go fix in #91.

This PR makes the cross-SDK `04-files` examples runnable against a live daemon and surfaces two latent bugs along the way (one fixed here, one filed as a follow-up).

What changes

7 examples rewritten to use real tempfiles + round-trip assertions, mirroring `antd-py/examples/04_files.py` and the post-#91 `antd-go/examples/03-files/main.go`:

• `antd-cpp/examples/04-files.cpp` — `std::filesystem::temp_directory_path` + `std::ofstream`/`std::ifstream`
• `antd-rust/examples/04-files.rs` — `std::env::temp_dir` + `std::fs`
• `antd-elixir/examples/04_files.exs` — `System.tmp_dir!` + `File.write!`/`File.read!`
• `antd-lua/examples/04-files.lua` — `/tmp/antd-lua-04-files` + `io.open`
• `antd-php/examples/04-files.php` — `sys_get_temp_dir` + `file_put_contents`/`file_get_contents`
• `antd-ruby/examples/04_files.rb` — `Dir.mktmpdir` + `File.write`/`File.read`
• `antd-zig/examples/04-files.zig` — `/tmp/antd-zig-04-files` + `std.fs.createFileAbsolute`

Each example writes a known file, calls `file_cost` → `file_upload_public` → `file_download_public`, asserts byte-equal round-trip, and cleans up the temp dir.

Ruby adapter expanded in `ant-dev/src/ant_dev/cmd_example.py` — the ruby adapter only dispatched `connect`/`data`/`chunks`, so the existing `04_files.rb` and `06_private_data.rb` files on disk were silently skipped. The sweep "passed" ruby in 5s without exercising files. Adapter now dispatches all 5.

Per-SDK signature bugs fixed that were hidden by the daemon-400 error in the original stubs:

• `antd-elixir`: `Antd.Client.file_cost/4` doesn't exist — only `/3`. Original called `file_cost(client, path, true, false)` (4 args).
• `antd-ruby`: `file_cost` takes 2 args, not 3. Original called `file_cost(path, true, false)`.
• `antd-php`: `fileCost()` returns an `UploadCostEstimate` object — original printed it with string interpolation, which is a PHP fatal. Now uses `$cost->cost` / `$cost->chunkCount` properties.

Surfaced (not fixed) — daemon directory-upload bug

The first iteration of these examples exercised `dir_upload_public` too. The daemon returns:

```
HTTP 500 Internal Server Error
Internal error: encryption failed: I/O error: Is a directory (os error 21)
```

The daemon appears to attempt encryption on the directory path itself rather than recursing into files. Verified in cpp/rust/lua/zig logs from the V2-299 sweep — file upload+download works, dir upload doesn't. This is latent because the prior stub examples errored at `file_cost` first, so nobody had exercised the directory route in v2.

To keep this PR scoped to the example fix, the dir-upload step has been removed from these examples. The daemon bug is filed separately for a real fix.

Test plan

• dev2 sweep with `ant dev example files -l ` for each affected SDK against `ant dev start --enable-evm`:

  SDK	Result	Time
  cpp	✅ PASS	5s
  rust	✅ PASS	6s
  elixir	✅ PASS	3s
  lua	✅ PASS	4s
  php	✅ PASS	3s
  ruby	✅ PASS	4s
  zig	✅ PASS	12s

  All 7 SDKs: real tempfile, real cost estimate, real upload, real download, real byte-equal assertion, clean exit.

• Reviewer: re-run the same sweep on a fresh devnet.

Why now

The dev2 e2e sweep is the validation gate for v1.0. Pre-this-PR, 6 of 9 graph-cleaned SDKs (post-#92) failed on `files` due to placeholder paths. After this PR, the file-example slice is green. The remaining v1.0 gaps are: the directory-upload daemon bug (separate ticket), the external-signer marquee example, and the SDKs that still have no examples/ dir at all (csharp/dart/swift/java/kotlin).

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