Send x-build-version header (arkd version compat, PR #960)

[arkanaai]

Background

arkd #960 (merged 2026-03-25) adds a new version compatibility check via a build version header:

• gRPC: x-build-version metadata key
• REST: X-Build-Version HTTP header
• Format: semver string, e.g. 1.0.0 or v1.0.0
• Enforcement: only major version is compared; missing/malformed headers are silently allowed through (opt-in today)

When the server enforces a minimum major version, clients sending an older major will receive error BUILD_VERSION_TOO_OLD (error code 48).

Required Changes

This SDK needs to send x-build-version (gRPC) / X-Build-Version (REST) on every request, populated with its own build/package version.

Tasks

• Determine the correct version source (package.json version, module version, build ldflags, etc.)
• Inject x-build-version into gRPC client metadata on every call
• Inject X-Build-Version into REST HTTP headers on every request
• Handle BUILD_VERSION_TOO_OLD error code (48) with a clear user-facing error message
• Add tests verifying the header is sent

Reference

• arkd PR: version in header arkd#960
• Version compat doc: added in arkd at docs/version-compat.md
• Error code 48 = BUILD_VERSION_TOO_OLD, gRPC status FAILED_PRECONDITION (code 9)

Notes

No header = server allows through (backwards compatible for now). But this should be implemented before any major version bump on arkd enforces it.