QOL changes for working with Ark submodule

[jmcphers]

Fixes #14407

Summary

Improves the developer experience around the ark submodule, which was awkward to work with when picking up in-flight Ark changes locally. Four related fixes:

1. Prefer building Ark from source over a stale prebuild on dev machines. The install-kernel resolution order now puts a local cargo build ahead of the older-prebuild fallback, so a developer with the rust toolchain gets the Ark their submodule points at. Downloading an older prebuild is now the last resort (for machines without Rust) and prints a stderr warning.

2. Include local builds in release bundles. install-kernel now writes a SUBMODULE_COMMIT sidecar next to the resolved binary. Because resources/ark/ ships in the packaged extension, the expected commit is available at runtime even in a release build with no git submodule.

3. Invalidate the npm cache on Ark version changes, so a stale Ark triggers a reinstall.

4. Warn on an Ark version mismatch. When the running Ark was built from a different commit than the one Positron expects, a toast appears once per session (mirroring the Kallichore supervisor warning).

The expected commit comes from the live submodule HEAD (dev) or the sidecar (release). It stays quiet when a mismatch is expected or unknowable (Ark reports no commit, no expected commit available, or the submodule has local commits on top of origin/main). It can be disabled via the new positron.r.kernel.warnOnVersionMismatch setting, and when a custom positron.r.kernel.path is set the toast names that path and points at the kernel-path setting.

The runtime info returned by the kernel now carries build_version and commit fields to support the warning, plumbed through JupyterLanguageInfo, KallichoreSession, and positron.d.ts.

Validation Steps

@:ark

Most of this change is build tooling (install-kernel.ts) and dev-time diagnostics not reachable by the standard test runners, so no automated tests were added. Validate manually:

Resolution order (dev):

1. On a machine with rust but no matching prebuild cached, run the positron-r post-install and confirm Ark is built from source rather than an older prebuild being downloaded.
2. Confirm resources/ark/SUBMODULE_COMMIT is written after install.

Version-mismatch warning:

1. Install an Ark whose commit differs from the submodule HEAD (e.g. force the older-prebuild fallback).
2. Start an R session and confirm a warning toast appears once, naming the running and expected commits.
3. Confirm no toast when the submodule HEAD has local commits on top of origin/main (active Ark development).
4. Set positron.r.kernel.warnOnVersionMismatch to false and confirm the toast is suppressed.
5. Set a custom positron.r.kernel.path to a mismatched build and confirm the toast names that path and opens the kernel-path setting.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:ark @:sessions @:console @:interpreter

Why these tags?

Tag	Source
@:critical	Always runs (required)
@:ark	PR description
@:sessions	Changed files
@:console	Changed files
@:interpreter	Changed files

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