bazel/wasm: upgrade wasmtime from 32.0.0 to 42.0.2

Additional changes required by the upgrade:

- Bump Rust toolchain from 1.86.0 to 1.91.0 (wasmtime 42 MSRV)
- Add PULLEY and ALL_ARCH feature flags to wasmtime.BUILD conf.h
  template substitutions (new in wasmtime 42)
- Remove wasmtime_config_async_support_set() call: async support is
  now implicit, the separate toggle was deprecated and removed from
  the C API (bytecodealliance/wasmtime#12371)
- Bump rules_rust 0.60.0 -> 0.70.0 and opt in to
  experimental_use_allocator_libraries_with_mangled_symbols so the
  mangled __rustc::* allocator shim symbols (introduced in rustc
  1.87+) are provided when a Rust staticlib is linked into a
  cc_binary via cc_common.link. Without this flag the link fails
  with undefined references to __rustc::__rust_alloc,
  __rustc::__rust_dealloc, etc. Background:
    - Issue:   bazelbuild/rules_rust#3459
    - Fix PR:  bazelbuild/rules_rust#3403
    - Landed in rules_rust 0.63.0 as an opt-in setting.
- Mirror the rustc tarball through vectorized-public to reduce
  dependence on static.rust-lang.org.
- Incidental: bump bazel_skylib 1.8.1 -> 1.8.2 and rules_shell
  0.4.1 -> 0.6.1 to silence direct-dependency version warnings
  pulled in by the new rules_rust.

Author: travisdowns

.bazelrc

@@ -17,6 +17,20 @@ build --@rules_go//go/config:pure
 build:gofips --//bazel:gofips=True --@rules_go//go/toolchain:sdk_name=go_sdk_with_systemcrypto
 build:gofips --@rules_go//go/config:pure=false
 
+#################
+# Rust settings #
+#################
+
+# Use rules_rust's Rust-side allocator_library that defines the
+# mangled `__rustc::*` allocator shim symbols (rust_alloc,
+# rust_dealloc, etc.). The default cc_allocator_library predates the
+# allocator-symbol mangling in rustc 1.87+ and leaves those symbols
+# undefined when a Rust staticlib is linked into a cc_binary via
+# cc_common.link. See bazelbuild/rules_rust#3459.
+build --@rules_rust//rust/settings:experimental_use_allocator_libraries_with_mangled_symbols=True
+
+# end of rust settings #
+
 # Improve caching and readability of filepaths that the compiler uses by not using
 # absolute paths, and instead use paths that are relative to the redpanda repo.
 # https://github.com/bazel-contrib/toolchains_llvm/pull/445#issuecomment-2605443516

MODULE.bazel

@@ -19,7 +19,7 @@ use_repo(
 
 bazel_dep(name = "gazelle", version = "0.45.0")
 bazel_dep(name = "abseil-cpp", version = "20250814.0")
-bazel_dep(name = "bazel_skylib", version = "1.8.1")
+bazel_dep(name = "bazel_skylib", version = "1.8.2")
 bazel_dep(name = "crc32c", version = "1.1.0")
 bazel_dep(name = "fmt", version = "12.1.0")
 bazel_dep(name = "googletest", version = "1.17.0")
@@ -52,7 +52,7 @@ bazel_dep(name = "rules_go", version = "0.59.0")
 bazel_dep(name = "rules_pkg", version = "1.0.1")
 bazel_dep(name = "rules_proto", version = "7.1.0")
 bazel_dep(name = "rules_python", version = "1.6.0")
-bazel_dep(name = "rules_rust", version = "0.60.0")
+bazel_dep(name = "rules_rust", version = "0.70.0")
 bazel_dep(name = "snappy", version = "1.2.1")
 bazel_dep(name = "yaml-cpp", version = "0.8.0")
 bazel_dep(name = "zlib", version = "1.3.1.bcr.6")
@@ -69,7 +69,7 @@ single_version_override(
     ],
 )
 
-bazel_dep(name = "rules_shell", version = "0.4.1", dev_dependency = True)
+bazel_dep(name = "rules_shell", version = "0.6.1", dev_dependency = True)
 
 bazel_dep(name = "protovalidate", version = "1.0.0")
 
@@ -350,7 +350,16 @@ rust.toolchain(
         "aarch64-unknown-linux-gnu",
         "x86_64-unknown-linux-gnu",
     ],
-    versions = ["1.86.0"],
+    # Prefer our S3 mirror for the rustc tarball (static.rust-lang.org is
+    # frequently slow from some POPs). Only the `rustc-*` component is
+    # mirrored; other components (rust-std, cargo, clippy, rustfmt,
+    # llvm-tools, rust-src) 404 on the mirror and fall through to the
+    # upstream URL automatically.
+    urls = [
+        "https://vectorized-public.s3.us-west-2.amazonaws.com/dependencies/{}.tar.xz",
+        "https://static.rust-lang.org/dist/{}.tar.xz",
+    ],
+    versions = ["1.91.0"],
 )
 use_repo(rust, "rust_toolchains")