Implement full module extension with toolchain, libc detection, and Gazelle manifest

[xangcastle]

Replaces the stub uv/unstable/extension.bzl (previously a one-liner alias to the
stable extension) with a complete, self-contained module extension that owns its own
toolchain download, platform detection, and Gazelle integration.

New features

Hermetic UV toolchain provisioning (uv/private/toolchain/)

• uv_repository: downloads a UV binary for a specific platform from GitHub releases
  with optional SHA256 verification
• uv_host_repository: auto-detects the host platform and provisions the correct
  binary
• uv_platform_repository: selects the right binary for the current Bazel execution
  environment
• uv_toolchains_hub: wires all per-platform repos into a single toolchain_type hub
  across 7 supported platforms (macOS arm/x86, Linux glibc/musl arm/x86, Windows x86)
• New uv_tool_toolchain rule and UvToolInfo provider in
  uv/private/toolchain/types.bzl

Host libc detection (uv/private/constraints/libc/)

• detect_libc() function that inspects ldd --version, musl dynamic linker presence,
  and /etc/os-release to distinguish glibc from musl at repository-fetch time
• libc_detector repository rule that writes the detected variant into a generated
  defs.bzl (LIBC_VARIANT) and config_setting targets

Gazelle manifest generation from uv.lock (uv/unstable/gazelle.bzl,
uv/unstable/lock_parser.bzl)

• gazelle_python_yaml_repository: repository rule that downloads wheels listed in
  uv.lock, inspects each wheel's top_level.txt (with directory-scan fallback) to map
  import names → package names, and emits a gazelle_python.yaml
• lock_parser.bzl: minimal uv.lock parser that extracts per-platform wheel URLs and
  SHA256 hashes without requiring a full TOML library
• Exposed as the new gazelle_manifest tag class on the unstable extension

Full uv module extension (uv/unstable/extension.bzl)

• Upgraded from uv = _uv alias to a complete module_extension with six tag classes:
  toolchain, declare_hub, project, unstable_annotate_packages, override_package,
  gazelle_manifest
• Delegates dependency resolution to the stable uv_impl while adding toolchain
  setup, libc detection, and Gazelle manifest generation on top

Tests

• //uv/unstable:inspect_wheel_test — 10 unit tests for the wheel inspection logic
  (top_level.txt, directory scan fallback, metadata exclusion, name normalisation)
• //uv/private/constraints/libc:detect_libc_test — 8 unit tests for the libc
  detection heuristics via a Python mirror of the Starlark logic
• //e2e/cases/uv-gazelle-unstable:test_gazelle_manifest — e2e test that exercises
  the new uv.gazelle_manifest() tag end-to-end and asserts the generated YAML contains
  the expected cowsay: cowsay mapping

Test plan

• bazel test //uv/unstable:inspect_wheel_test
• bazel test //uv/private/constraints/libc:detect_libc_test
• bazel test //... in the root workspace (all existing tests continue to pass)
• cd e2e && bazel test //cases/uv-gazelle-unstable:test_gazelle_manifest
• cd e2e && bazel test //... (full e2e suite)

[aspect-workflows]

Bazel 8 (Test)

2 test targets passed

Targets

//uv/private/constraints/libc:detect_libc_test [k8-fastbuild-ST-32abbbcff250] 200ms
//uv/unstable:inspect_wheel_test [k8-fastbuild-ST-32abbbcff250]               351ms

Total test execution time was 551ms. 120 tests (98.4%) were fully cached saving 57s.

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Bazel 9 (Test)

2 test targets passed

Targets

//uv/private/constraints/libc:detect_libc_test [k8-fastbuild-ST-32abbbcff250] 169ms
//uv/unstable:inspect_wheel_test [k8-fastbuild-ST-32abbbcff250]               387ms

Total test execution time was 556ms. 119 tests (98.3%) were fully cached saving 1m 4s.

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Bazel 8 (Test)

e2e

1 test target passed

Targets

//cases/uv-gazelle-unstable:test_gazelle_manifest [k8-fastbuild-ST-1a8602e72ef0]429ms

Total test execution time was 429ms. 54 tests (98.2%) were fully cached saving 60s.

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Bazel 9 (Test)

e2e

1 test target passed

Targets

//cases/uv-gazelle-unstable:test_gazelle_manifest [k8-fastbuild-ST-1a8602e72ef0]568ms

Total test execution time was 568ms. 54 tests (98.2%) were fully cached saving 55s.

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Bazel 8 (Test)

examples/uv_pip_compile

All tests were cache hits

1 test (100.0%) was fully cached saving 444ms.

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