fix stack extra: distribution name is arc-stack, allow direct refs

Two adjustments so `uv sync --extra stack` resolves:

- The dependency spec must use the upstream package's distribution
  name (arc-stack, declared in github.com/ArcInstitute/stack
  pyproject), not its Python import name (stack).

- Hatchling refuses direct URL references in dependencies unless
  explicitly opted in, so add [tool.hatch.metadata]
  allow-direct-references = true.

Verified: `uv sync` clean, `uv sync --extra stack` builds arc-stack
from the GitHub repo.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lagillenwater

pyproject.toml

@@ -28,7 +28,7 @@ ml = [
     "transformers>=4.40",
 ]
 stack = [
-    "stack @ git+https://github.com/ArcInstitute/stack.git",
+    "arc-stack @ git+https://github.com/ArcInstitute/stack.git",
 ]
 dev = [
     "pytest>=8.0",
@@ -43,6 +43,9 @@ dev = [
 requires = ["hatchling"]
 build-backend = "hatchling.build"
 
+[tool.hatch.metadata]
+allow-direct-references = true
+
 [tool.hatch.build.targets.wheel]
 packages = ["src/fmharness"]