FN-5761: add workflow IR parsing and serialization contract

Implement the v1 workflow IR contract in core with validation, serialization, and coverage.

- add workflow IR type definitions, parser, serializer, and structured validation errors in @fusion/core
- export workflow IR APIs from core index and add comprehensive unit tests for valid/invalid shapes and metadata parsing
- document the workflow IR contract in workflow step docs and include release tracking via changesets, including plugin-sdk DOM lib prerequisite for Windows compilation

Files changed:
 .changeset/fn-5761-workflow-ir-contract.md       |   5 +
 .changeset/windows-release-plugin-sdk-dom-lib.md |   5 +
 docs/workflow-steps.md                           |  24 +++
 packages/core/src/__tests__/workflow-ir.test.ts  |  70 +++++++
 packages/core/src/index.ts                       |  15 ++
 packages/core/src/workflow-ir-types.ts           | 103 ++++++++++
 packages/core/src/workflow-ir.ts                 | 244 +++++++++++++++++++++++
 packages/plugin-sdk/tsconfig.json                |   7 +-
 8 files changed, 472 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-5761

Fusion-Task-Lineage: 2cd5491e-89c8-418c-a913-76d0c13b466c

packages/plugin-sdk/tsconfig.json

@@ -3,7 +3,12 @@
     "target": "ES2022",
     "module": "NodeNext",
     "moduleResolution": "NodeNext",
-    "lib": ["ES2022"],
+    // DOM is required because @fusion/core exports its types as raw src/*.ts
+    // (exports."." -> ./src/index.ts), so this project recompiles core's source
+    // here. Core relies on the DOM lib (default lib via tsconfig.base) for the
+    // global fetch `Response` type (.ok/.status/.json); without DOM those
+    // members resolve inconsistently across platforms (Windows CI build failed).
+    "lib": ["ES2022", "DOM"],
     "types": ["node"],
     "outDir": "dist",
     "rootDir": "src",