feat(codegen): register Swift client plugin

[scarmuega]

Summary

• register swift-client as a first-party built-in codegen plugin
• pass swift-client through to tx3c as the built-in template name
• raise the tx3c compatibility floor to the published 0.25.0 release
• cover parsing, suggestions, default configuration, template delegation, and the updated version gate

Verification

• cargo fmt --all -- --check
• cargo build --workspace --locked
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --all-targets --all-features --locked

All checks pass locally. Cargo reports only the existing future-incompatibility notice for chumsky v1.0.0-alpha.7.

[scarmuega]

Rework delivered at exact head b59da2d. Merged current main (07a52f2) without rebasing and resolved the overlapping Java/Swift registration so both built-in clients remain in the registry, CLI help, examples, stable-name/suggestion coverage, contract tests, and the single tx3c 0.25.0 compatibility-floor rationale. Exact-head local verification passed: cargo fmt --all -- --check; cargo build --workspace --locked; cargo clippy --workspace --all-targets --all-features --locked -- -D warnings; cargo test --workspace --all-targets --all-features --locked (81 unit, 11 CLI, 18 contract; 110 total). Only the existing non-blocking chumsky v1.0.0-alpha.7 future-incompatibility notice remains. GitHub CI is running on this pushed head.

[scarmuega]

Merge observed.

Plan: plans/swift-sdk-trix-registration.md
QA-approved head: efaa2bdedf26a8c983baafcbaa97b7b70972bfb1

Trellis will verify the human merge evidence, adopt the domain pin, and retire the plan. No further merge action is needed.