Add a lint to keep internal/external Vela versions in sync

[rascani]

Problem

The Vela version is declared in three places that can drift apart, and currently do: executorch/backends/arm/PACKAGE (internal override), backends/arm/requirements-arm-ethos-u.txt (5.1.0), and pyproject.toml. Nothing keeps them consistent, so a silent mismatch is a latent "works here, breaks there."

What to do

Add a soft lint that checks the three pins against each other. It must be advisory, not blocking: the gap is intentional today, and there are transient windows during an in-flight import where the files legitimately disagree. Normalize the two version grammars (Buck 5.0.0._git_… / v2026.05.0 vs pip 5.1.0 / 2026.5.0) and encode the allowed internal-vs-OSS exception (with a documented reason and tracking task). Because arm/PACKAGE is visible in open source, its drift from the OSS pin is also a signal that we should trigger a pre-merge import.

Steps

• Parse and normalize the pins in the three files.
• Encode the allowed internal-vs-OSS exception (documented reason + tracking task).
• Make it advisory — warn, never block the DiffTrain or the merge queue.
• Emit an "import due" signal when arm/PACKAGE diverges from the OSS pin.

Done when

The three pins cannot silently diverge without a warning, and an intended divergence is documented rather than flagged as an error.