feat: add IR tooling — gen eject codegen and the crucible CLI

[joshua-temple]

What this change does

Adds the headless IR tooling — the backbone a visual editor will eventually sit on — as two new, independently-versioned modules:

• gen — an "eject" codegen. gen.Eject turns a machine's IR into typed Go stub source: a context type derived from the IR's ContextSchema, a panic("TODO")-bodied stub for every behavior the IR references (typed to the engine's exact guard/action/assign/service signatures), and a Provide(reg) that registers each stub by name so the IR binds. Output is gofmt'd and deterministic. A test writes the generated source into a temp module and go builds it against state, proving the stubs compile and wire.
• cmd/crucible — an IR command-line tool with lint (analysis), render (Mermaid/DOT), diff (evolution semver classification), validate, and eject (via gen). Structural commands work without real behaviors by binding deterministic no-op stubs for every referenced name, so a definition can be inspected, rendered, and analyzed straight from its IR.

Why

This is the contract-consuming half of crucible's IR story: it exercises the IR, palette, analysis, and evolution surfaces, which is what proves those contracts are sufficient before the 1.0 freeze. Both modules version independently and are not part of the state v1 frozen surface.

Relates to: 1.0 IR tooling

Checklist

• Commits are signed off (git commit -s) per the DCO
• Conventional commit messages (type: subject)
• mage check equivalent passes locally — both new modules pass go build, go test, and golangci-lint; the workspace builds.
• Tests added/updated for the change
• Public API documented (godoc + module READMEs/changelogs)