feat: delegate low-level ops to the tx3c binary; drop tx3-* crates

[scarmuega]

What

trix no longer links tx3-lang/tx3-tir. Every low-level Tx3 operation is delegated to the tx3c binary as a subprocess; a dependent tool's CLI surface is the versioned contract (driver pattern, cf. cargo↔rustc). cargo tree -i tx3-lang / -i tx3-tir now both error.

• spawn/compat.rs: per-tool version matrix + ensure_supported(), routed through every external-CLI chokepoint (tx3c, cshell, dolos). Inclusive lower bound; exclusive upper bound derived as the next major (a breaking CLI change must come with a major bump, so same-major releases are accepted). Distinct too-old / too-new messages. TX3_SKIP_COMPAT_CHECK escape hatch.
• check / inspect tir: delegate to tx3c (build --diagnostics-format json, build --emit tir-json, decode --emit tir-json); diagnostics reconstructed from the JSON contract (terminal output unchanged).
• codegen: legacy in-process module removed; the tx3c-delegating path is the only one. Output nests under gen/<project>/.
• Cargo.toml: drop tx3-lang/tx3-tir, add semver.
• design/004-toolchain-delegation.md: the general "trix drives dependent CLIs as subprocesses" decision (tx3c/dolos/cshell), tool-agnostic.

⚠️ Release ordering — do not merge/tag before tx3 0.18.0

• Depends on feat(tx3c): add decode verb, emit tir-json and diagnostics-format tx3#324 (the tx3c surface). Compat window is tx3c >= 0.18.0, < 1.0.0; that surface ships in tx3 0.18.0. Confirm the lower bound against the actually published tx3 version before tagging trix.
• CI must export TX3_SKIP_COMPAT_CHECK=1 until a tx3c >= 0.18.0 is installed (a locally built tx3c still reports the pre-release version). The e2e harness already sets it; a workflow-level export is needed for any path that runs the built trix directly. Remove once tx3 0.18.0 is in CI's toolchain.

Known follow-up (accepted)

CURRENT_CODEGEN_VERSION is flipped to codegen-v1beta0 for the built-in SDK plugins (web/rust/python/go). The SDK repos do not yet ship codegen-v1beta0 bindgen templates — a knowingly-accepted gap to close via a fast follow-up in those repos before a user-facing release.

Out of scope / handoff

The tx3-lang/up channel must pair tx3 0.18.0's tx3c with the next trix so tx3up installs a matched set. Not in this repo.

Verification

• cargo build; cargo tree -i tx3-lang / -i tx3-tir both error.
• cargo test: 20 passed, 1 failed — the failure is devnet_starts_and_cshell_connects (dolos binary absent in this env), pre-existing and unrelated; expected green in CI where dolos is present.
• Compat boundaries verified: tx3c 0.17.0 → too-old; 0.18.0 / 0.19.0 / 0.99.0 → ok; 1.0.0 → too-new.
• inspect tir (project + interface) byte-identical to direct tx3c; check reproduces analyzer codes/messages and exit status.

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