metrics: show TiCDC architecture mode in build info

[wlwilliamx]

Is your feature request related to a problem?

TiCDC (new architecture repo) uses a single cdc binary that can start in either:

• new architecture mode (--newarch / TICDC_NEWARCH=true / newarch=true in config), or
• old architecture mode (delegates to TiFlow).

Today the monitoring "Build Info" panel only shows build metadata (version/hash/build time/kernel type), so operators cannot tell which architecture mode an instance is actually running.

Describe the feature you'd like

Expose the runtime architecture mode in monitoring by:

• adding a ticdc_arch label to ticdc_server_build_info with value newarch or oldarch, and
• showing ticdc_arch in the Grafana Build Info table.

Describe alternatives you've considered

• Rely on logs (not always available in the monitoring workflow).
• Infer from the presence/absence of certain metrics (can be brittle and changes over time).

Teachability, Documentation, Adoption, Migration Strategy

• Update the TiCDC build info dashboard query to include ticdc_arch.
• Keep backward compatibility for old-arch instances that do not export ticdc_server_build_info by falling back to another always-present metric (if needed).