fix(hub-repo): register per-platform toolchain() rules instead of aliases

[avrabe]

Problem

Downstream projects using rules_verus via:

```starlark
register_toolchains("@verus_toolchains//:all")
```

hit Bazel's ambiguity rule — `:all` is both a wildcard and the name of an existing alias rule in the hub repo. Bazel picks the alias interpretation, which points to only the first platform's underlying target (not even a `toolchain()` rule), so no toolchains are registered:

```
WARNING: The target pattern '@verus_toolchains//:all' is ambiguous:
':all' is both a wildcard, and the name of an existing alias rule;
using the latter interpretation
ERROR: While resolving toolchains for target //:rivet_specs_verify:
No matching toolchains found for types
```

This is blocking Verus CI on rivet (https://github.com/pulseengine/rivet/actions/runs/24649838100/job/72070649411).

Fix

Change the hub repo's generated BUILD.bazel from an alias-based layout to one proper `toolchain()` wrapper per supported platform:

• Each wrapper references the underlying `verus_toolchain_info` provider.
• Each wrapper has matching `exec_compatible_with` / `target_compatible_with` constraints.
• The conflicting alias named `all` is removed.

With no alias shadowing `:all`, Bazel's wildcard-package interpretation takes over and registers all per-platform `toolchain()` rules at once. Native toolchain resolution then selects the correct one based on the exec platform.

Scope

• 1 file changed (`verus/extensions.bzl`)
• 44 insertions, 55 deletions
• No changes to the per-platform repo emitted by `verus/private/repo.bzl` — the `verus_toolchain_info` provider it already exports works as-is.
• Backwards-compatible for any consumer using `register_toolchains("@verus_toolchains//:all")`.

Test plan

• `cd rivet/verus && bazel test //:rivet_specs_verify` passes toolchain resolution on Linux CI
• Local smoke tests still need to be run (no bazel on my machine)

🤖 Generated with Claude Code