feat(codegen): render built-in plugins from tx3c's built-in templates

[scarmuega]

Summary

tx3c now ships the first-party templates itself and selects them by name (tx3-lang/tx3#360, tx3-lang/tx3#361). A built-in plugin name such as rust-client is also the name of its tx3c template. The built-in plugins ts-client, rust-client, python-client and go-client therefore call tx3c codegen --template <plugin name>. They no longer download each SDK repo's .trix/client-lib at the codegen-v1beta0 tag.

• A generated client always matches the installed tx3c, and built-in codegen needs no network access.
• Custom { repo, path, ref } plugins keep the download and pass the downloaded directory to the same --template flag.
• CURRENT_CODEGEN_VERSION and the built-in repo, path and ref table are removed. There is no mapping from plugin to language either, because the plugin name is passed straight through.
• The tx3c compatibility floor rises to 0.24.0. That assumes 0.24.0 is the release that ships the built-in templates from tx3#360 and tx3#361.
• The two example projects replace their stale [[bindings]] blocks with [[codegen]] plugin = "ts-client".

Merge order

Release tx3c with tx3#360 and tx3#361 before releasing trix with this PR. A trix release that includes this change rejects any tx3c older than 0.24.0. If the tx3c release gets a different version number, update the floor in src/spawn/compat.rs before merging.

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --all-targets --all-features --locked, which passes, including a new contract test. It checks that built-in plugins pass their name as --template and never download templates.
• End to end: trix init followed by trix codegen --plugin go-client, using the real tx3c built from tx3#361, rendered go.mod, protocol.go and README.md with no download. The compatibility gate was skipped, because that unreleased build still reports 0.23.0.

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