fix(antd-elixir): print cost-estimate fields in examples to avoid stringification crash

[Nic-dorman]

Fixes #67.

The Elixir examples interpolated whole Antd.UploadCostEstimate structs into strings, which raises Protocol.UndefinedError (the struct has no String.Chars impl):

** (Protocol.UndefinedError) protocol String.Chars not implemented for
   Antd.UploadCostEstimate (a struct)
    02_data.exs:19: (file)

Same shape as the PHP fix in #74. The SDK return types are correct (data_cost/2 etc. correctly return {:ok, %UploadCostEstimate{}}); only the example interpolations are wrong.

Changes

• 02_data.exs — rewritten end-to-end to match the Python / Rust / PHP examples: estimate-then-store ordering, print individual estimate fields, add a round-trip assertion and an "OK" marker so the example self-verifies.
• 04_files.exs:28 — single-line fix on the file_cost print.
• 05_graph.exs:30 — single-line fix on the graph_entry_cost print.

Test plan

• cd antd-elixir/examples && elixir 02_data.exs against a local devnet — exits 0, prints estimate fields, address, "Public data round-trip OK!"
• Verified via the cross-SDK e2e harness used to find this bug
• 04_files.exs and 05_graph.exs visually verified only — they have independent unrelated issues (e.g. 04 calls file_cost/4 but the SDK exports file_cost/3) that should be addressed separately

Sample output

```
Estimate: 16 bytes in 3 chunks, storage 0 atto, gas 0 wei, mode single
Stored at address: 01b06330e4f4a8c565087e4087caabf860bdadbe96b7b8f0447493b0081c2335
Actual cost: atto tokens
Retrieved: Hello, Autonomi!
Public data round-trip OK!
```

(Several pre-existing compile warnings about Antd.V1.FileService.Stub.get_file_cost/2 being undefined surface during the run — that's unrelated to this PR and worth its own ticket.)