fix(uv): restore Bazel 7 compatibility by replacing set() with a bazel_features-aware compat shim

[xangcastle]

Problem

Users on Bazel 7 hit a load-time error in graph_utils.bzl and
uv_project/repository.bzl because set() is a Starlark built-in
that was only introduced in Bazel 8. Any workspace still on Bazel 7
fails immediately with:

  name 'set' is not defined

Solution

Adds uv/private/compat.bzl, a thin shim that uses bazel_features
(already a project dependency) to select the right implementation at
repository evaluation time:

• Bazel 8+: delegates to the native set() constructor — O(1)
  membership, zero overhead.
• Bazel 7: falls back to a plain dict used as a set, which is the
  standard Starlark idiom and fully compatible.

Iteration (for x in s) and membership (x in s) work identically on
both a native set and a dict, so call-site ergonomics are unchanged.

Affected files

• uv/private/compat.bzl — new shim (new_set / set_add)
• uv/private/extension/graph_utils.bzl — was line 25
• uv/private/uv_project/repository.bzl — was line 93

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Changes are visible to end-users: no

Test plan

• Covered by existing test cases

[aspect-workflows]

✨ Aspect Workflows Tasks

📅 Mon Jun 1 17:14:30 UTC 2026

✅ 7 successful tasks

• ✅ buildifier · ⏱ 21.3s · 🐙 GitHub Actions · ☑️ Check
  💬 Format complete (clean)
• ✅ gazelle · ⏱ 37.4s · 🐙 GitHub Actions · ☑️ Check
  💬 Gazelle complete (clean)
• ✅ test (test-e2e-bazel-8) · ⏱ 28m 40s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (137/137 passed)
• ✅ test (test-e2e-bazel-9) · ⏱ 28m 47s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (132/132 passed)
• ✅ test (test-examples-uv_pip_compile-bazel-8) · ⏱ 36.1s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (1/1 passed · 1 cached)
• ✅ test (test-root-bazel-8) · ⏱ 20m 49s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (185/185 passed)
• ✅ test (test-root-bazel-9) · ⏱ 22m 41s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (184/184 passed)

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