feat(tx3c): select built-in templates by name with --template

[scarmuega]

Summary

Follow-up to #360, before any release ships --language. A template is one use case in one language, such as rust-client. It is therefore the single dimension tx3c codegen selects. Language and use case are not separate flags.

• One flag. --template <name-or-dir> replaces the unreleased --language. A bare name, with no path separator and no leading ., selects a built-in template when one has that name. Anything else is a custom template directory, exactly as before, so every released caller of --template <dir> keeps working. ./name forces a directory.
• Built-in templates are ts-client, rust-client, python-client and go-client. The names match trix's plugin names, so trix can pass them straight through. Their files move to bin/tx3c/templates/<name>/ and are listed in a name-keyed registry, codegen/templates.rs.
• Backends don't own templates. A template picks its language backend through the helpers it calls, for example declarations tii "rust". Backend::client_templates() is removed.
• Adding a use case. A new use case in an existing language, such as a Rust server stub, is one template directory plus one registry entry. The CLI doesn't change.

tx3c codegen --tii p.tii --template rust-client --output out   # built-in
tx3c codegen --tii p.tii --template ./my-server --output out   # custom directory

Tests and CI

• A unit test fails if the registry and the template directories disagree. New tests cover name and path resolution and both error messages.
• Golden output moves from expected/<language> to expected/<name> with identical content.
• The codegen compile CI job now runs per template.

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test -p tx3c --locked, with 25 unit tests plus the golden corpus.
• Locally, codegen-compile-check.sh passed for rust-client and ts-client against the published 0.15.0 SDKs. CI covers python-client and go-client.

tx3-lang/trix#132 is updated to pass --template <plugin name> for built-in plugins.

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