fix(antd-swift): port to Linux + populate cost-estimate fields

[Nic-dorman]

Summary

antd-swift/README.md says the SDK is macOS-only, but the swift.org Linux toolchain (6.0.x) builds and round-trips against antd end-to-end after a small set of upstream-fixable patches. This PR collects them and adds a snake-case decoding fix for the cost estimate that was hiding behind the macOS-only framing.

Verified end-to-end on Ubuntu 24.04 / Swift 6.0.3:

swift build && swift run AntdExamples 2
…
=== Example 02: Public Data ===
Estimate: 24 bytes in 3 chunks, storage 43945312500000000 atto, gas 150000000000000 wei, mode single
Stored at address: b2898da4c31d2701da63eebde490047dbb4cc2b8eeb5d596009fe633ecafd20b
Retrieved: Hello, Autonomi network!
Public data round-trip OK!

Changes

Package.swift — bump grpc-swift to 2.x

grpc-swift-protobuf 1.x requires Swift tools 6.0+, conflicting with the previous from: "1.23.0" pin on grpc-swift (1.x line, 5.x tooling). Bumps grpc-swift to 2.0.0 and adds the new grpc-swift-nio-transport package — GRPCNIOTransportHTTP2 moved out of the main grpc-swift repo in 2.x.

Sources/AntdSdk/AntdRestClient.swift — three fixes

1. FoundationNetworking import — Linux splits URLSession out of Foundation. Guarded with #if canImport(FoundationNetworking) so macOS/iOS are unaffected.

2. PUT-response cost is optional — daemon currently returns {address, chunks_stored, payment_mode_used} for POST /v1/data/public / /v1/data/private / /v1/chunks without cost. The previous let cost: String decoder fataled. Fall back to empty-string downstream, matching antd-php's pattern and what fix(antd-kotlin): make put-response cost optional; ship missing gradle wrapper #77 just shipped for kotlin.

3. CostDTO snake_case decoding — the bug hidden behind the macOS-only framing. JSONDecoder.snakeCase uses keyDecodingStrategy = .convertFromSnakeCase, so JSON file_size is mapped to expected Swift property fileSize. The DTO had snake_case property names, so decoding silently nilled them out and dataCost / fileCost always returned zero for fileSize / chunkCount / gas / mode. Rename to camelCase so the decoder matches.

Sources/AntdSdk/Errors.swift — override on subclass inits

Swift 6's stricter init resolution flagged 9 error subclass initializers as missing override against AntdError.init(_:statusCode:).

Sources/AntdExamples/Main.swift — Linux-safe minimal example

The previous bundled example used SecRandomCopyBytes (macOS-only Security.framework) and had arity bugs in several of the per-example helpers. Replace with a minimal Linux-portable example 02 that exercises the public data round-trip and exits non-zero on mismatch — enough for cross-SDK e2e validation. Restoring the full example suite is a separate task once SecRandomCopyBytes is replaced and the arity bugs are fixed.

Test plan

• cd antd-swift && swift build clean on Ubuntu 24.04 / Swift 6.0.3 (one cosmetic warning: dependency 'grpc-swift' is not used by any target — only the nio-transport sub-package is consumed)
• swift run AntdExamples 2 against a local devnet — round-trip OK, real estimate fields populated
• Cross-SDK e2e harness: 15/15 SDKs pass with swift no longer skipped

Companion PR

#86 wires swift run AntdExamples N into the ant dev example dispatcher and removes the skip_reason that was silently auto-skipping swift. Either PR can land first; without these antd-swift patches, the dispatcher's swift adapter will fail at compile.

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