fix(antd-zig): pass payment_mode to dataPutPublic/dataPutPrivate in examples

[Nic-dorman]

Fixes #70.

examples/02-data.zig and examples/06-private-data.zig were calling the SDK's put methods with one arg short:

examples/02-data.zig:16:34: error: member function expected 2 argument(s), found 1
    const put_result = try client.dataPutPublic(message);
                           ~~~~~~^~~~~~~~~~~~~~
src/antd.zig:181:9: note: function declared here
    pub fn dataPutPublic(self: *Client, data: []const u8, payment_mode: ?[]const u8) !PutResult {

Pass null for the optional payment-mode (mirrors 04-files.zig, which already passes null to fileUploadPublic).

Affected:

• 02-data.zig:16 — dataPutPublic
• 06-private-data.zig:16 — dataPutPrivate

03-chunks.zig (chunkPut is single-arg) and 04-files.zig (already correct) are unchanged.

Test plan

• Type-checks the SDK API surface with the new arity (the compiler error from the wrong arity is gone)
• End-to-end run currently blocked by an unrelated Zig-version mismatch in src/json_helpers.zig / src/antd.zig itself: the source uses .empty (Zig 0.15-dev) AND http.Client.open (Zig 0.14.x), which don't coexist in any released Zig. Filed separately as antd-zig source uses Zig APIs from incompatible versions; doesn't compile on any released Zig #78. Once that lands, zig build run-02-data should green up.

Out of scope (worth a separate ticket)

examples/05-graph.zig calls graphEntryPut, graphEntryGet, graphEntryExists, and graphEntryCost, none of which exist on Client in src/antd.zig — looks like the entire graph example is targeting an API surface that hasn't landed yet. Not touching it here.