feat(compat): enforce project-declared minimum tx3c version from trix.toml

[scarmuega]

Summary

Adds an optional [toolchain] table to trix.toml so a protocol can declare the minimum tx3c version it requires:

[toolchain]
tx3c = "0.22.0"

trix enforces it through the existing version-gating path (spawn::compat), which already probes tx3c --version before each invocation. A project-declared minimum is a lower bound only: it raises the floor of trix's built-in COMPAT_MATRIX support window but never relaxes its upper bound, so a tx3c that is too old for the protocol is rejected before it is ever spawned.

Why the manifest (not an in-source pragma)

An in-source pragma tx3c "X.Y.Z" was prototyped and rejected. A minimum-version requirement is a tooling/build concern, not a language one, and the registry's primary artifact is the lowered TII (the .tx3 is informative) — a source never travels without its trix.toml. This mirrors Cargo's rust-version, and checking before spawn avoids any chance of an old tx3c choking on newer syntax before it can report the requirement.

Changes

• config/model.rs — ToolchainConfig { tx3c: Option<String> } + optional toolchain on RootConfig (skipped when absent).
• spawn/compat.rs — register_project_requirements + folding the project floor into the existing check; pure, testable helpers (evaluate, collect_project_mins). A malformed version string in [toolchain] fails fast.
• main.rs — register the project's declared minimums at the run_scoped_command chokepoint, before any tool spawn.
• init.rs — updated config literals.

Tests

7 new unit tests in spawn::compat: project-min as a lower bound; matrix window; project-min raising the floor above the matrix; matrix upper bound still applying when the project min is satisfied; manifest parse / absent-table / invalid-version. Full suite green; clippy clean.

Note: covers the pure decision logic and manifest parsing via unit tests; not exercised against a live installed tx3c (the subprocess plumbing is the pre-existing ensure_supported path).

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