fix(antd-lua): add missing discover module to rockspec; fix example cost-table concat

[Nic-dorman]

Fixes #68.

Two-part fix:

1. Rockspec gap (the headline bug). antd-scm-1.rockspec declared only 5 of the 6 .lua modules under src/antd/ — antd.discover was missing. After luarocks make, require("antd") blew up immediately because client.lua:10 does require("antd.discover") and there was no installed file for it:

```
module antd.discover not found:
no file '/usr/local/share/lua/5.4/antd/discover.lua'
no file '/home/nic/.luarocks/share/lua/5.4/antd/discover.lua'
...
```

One-line rockspec addition.

2. Example cost-table stringification. Once require worked the example then crashed on:

```
examples/02-data.lua:34: attempt to concatenate a table value (local 'cost')
```

Client:data_cost() returns a Lua table {cost, file_size, chunk_count, estimated_gas_cost_wei, payment_mode}, but the example concatenated the whole table to a string. Same shape as #66 (PHP) and #67 (Elixir). 02-data.lua is reworked end-to-end to match the Python/PHP/Elixir layout (estimate first, then store, then assert round-trip + OK marker). 04-files.lua:43 and 05-graph.lua:50 get single-line fixes.

Test plan

• luarocks --local --lua-version=5.4 make installs all 6 modules in ~/.luarocks/share/lua/5.4/antd/
• lua5.4 examples/02-data.lua against a local devnet — exits 0, prints estimate + address, "Public data round-trip OK!"
• Verified via the cross-SDK e2e harness
• 04-files.lua and 05-graph.lua are visually verified only — both have unrelated upstream issues (e.g. 04-files references a result.storage_cost_atto field whose presence I didn't verify here)

Linux/Lua-5.4 footnote

For Linux contributors: Ubuntu's luarocks package defaults to Lua 5.1. To install for 5.4 specifically: luarocks --local --lua-version=5.4 make (plus apt install liblua5.4-dev). Worth a one-line mention in antd-lua/README.md but out of scope here.