emsdk: use Python 3.12 and resolve interpreter without hardcoding (fix x86 build, stop patching bazel/MODULE.bazel)

[phlax]

Problem

bazel-registry/modules/emsdk/4.0.23.envoy/patches/emsdk_bzlmod.patch hardcodes a specific hermetic Python (python_3_13) and a fragile interpreter path. This currently fails the build on x86_64 (and would likely fail on arm too — but the observed failure is x86):

1. The emscripten toolchain shell scripts hardcode external/rules_python~~python~python_3_13_x86_64-unknown-linux-gnu/bin/python3. This uses the stale ~~ canonical-repo separator — current Bazel uses ++/+ (e.g. rules_python++python+python_3_13), so this path does not resolve.
2. Root MODULE.bazel / envoy standardize on Python 3.12. The patch forcing python_3_13 drags in a python_3_13 toolchain repo that is not visible / does not resolve from @@emsdk+:

ERROR: no such package '@@[unknown repo 'python_3_13' requested from @@emsdk+ ...]//':
No repository visible as '@python_3_13' from repository '@@emsdk+'
... referenced by '@@emsdk+//emscripten_toolchain:common_files_emscripten_linux'

These are arch-independent failures (stale repo syntax + non-resolving python_3_13 repo), not arm-specific.

Hardcoded spots in the current patch

• bazel/MODULE.bazel: adds use_repo(python, "python_3_13") — should not be in the patch at all (see below).
• bazel/remote_emscripten_repository.bzl: adds "@python_3_13//:files" to toolchain srcs.
• bazel/emscripten_toolchain/emar.sh, emcc.sh, emcc_link.sh: replace python3 with the hardcoded external/rules_python~~python~python_3_13_x86_64-.../bin/python3 path.

Required changes (confined to bazel-registry/modules/emsdk/)

1. Switch to Python 3.12 (match root/envoy default); remove all python_3_13 references.
2. No hardcoding: resolve the interpreter from the resolved rules_python toolchain / runfiles rather than a literal external/... path with a baked-in repo-name/separator/arch — so it is robust to Bazel canonical-repo syntax (~~ vs ++), python minor version, and architecture. Preserve the original hermetic intent (bazel-provided python, not system python3), just make it resolve correctly and tie it to 3.12.
3. Patch must not touch bazel/MODULE.bazel. Python config (python.toolchain / use_repo) belongs in the registry MODULE.bazel (bazel-registry/modules/emsdk/4.0.23.envoy/MODULE.bazel), not injected into upstream bazel/MODULE.bazel. Rem