refactor[next]: merge OTF definitions into stages, extract artifacts module

[egparedes]

Description

Second PR of the otf-toolchain-split stack (on top of #2736). Pure module restructuring — no class is renamed, no behavior changes:

• Merge otf.definitions into otf.stages: the definition-stage TypeVars/aliases (IRDefinitionT, ArgsDefinitionT, ConcreteProgramDef, CompilableProgramDef) and the step contracts (TranslationStep, CompilationStep) now live in stages, which becomes the single DSL-aware vocabulary module. otf/definitions.py is deleted.
• Extract otf.artifacts: the DSL-agnostic artifact models (ProgramSource, BindingSource, ExtensionSource, BuildSystemProject, ExecutableProgram, CompilationArtifact), all code-spec classes (otf/code_specs.py is deleted), and format_source (moved out of binding.interface, which removes the interface -> code_specs import edge and with it the last cycle risk in the bottom layer).
• Move the ConcreteArtifact pair type from otf.toolchain to the DSL-neutral bottom module otf.workflow, with no re-export left behind.
• Drop nominal step-protocol bases from CPPCompiler, DaCeTranslator, and DaCeCompiler — the step protocols are structural (Protocol), so subclassing them was never required; explicit mixin bases are kept where .replace() is actually used.

Net effect: otf.workflow, otf.artifacts, otf.compilation, otf.runners, and otf.binding are now formally IR-free (no imports from ffront/iterator), preparing the later split of otf into toolchain core and build infrastructure. Persistent cache keys are unchanged for all cached steps (fingerprints use fully-qualified class names; no cached class moved).

Breaking changes

No compatibility re-exports are left behind — a clean break rather than a deprecation period:

• gt4py.next.otf.definitions no longer exists; import those names from gt4py.next.otf.stages.
• gt4py.next.otf.code_specs no longer exists; import the code-spec classes from gt4py.next.otf.artifacts.
• The artifact models are no longer reachable as otf.stages.<Name> — use otf.artifacts.<Name>.
• otf.toolchain.ConcreteArtifact moves to otf.workflow.ConcreteArtifact.

Requirements

• All fixes and/or new features come with corresponding tests. (Import moves only; existing suites run against the new layout.)
• Important design decisions have been documented in the appropriate ADR inside the docs/development/ADRs/ folder. (No decision changed here; the naming/pipeline ADR lands later in this stack.)

https://claude.ai/code/session_01R8zRtFMhdJ8c96XJYCXkRk