Extension point: custom breadcrumb

[hpehl]

Parent: #320

Summary

Allow resources to customize breadcrumb segment rendering or add breadcrumb actions. Currently, ResourceBreadcrumb renders segments as key=value with a copy-to-clipboard action on the last segment, with no way to customize the display.

Motivation

• Custom segment rendering — show friendly names instead of raw key=value (e.g., "Datasources > ExampleDS" instead of subsystem=datasources > data-source=ExampleDS)
• Extra breadcrumb actions — besides copy-to-clipboard, add resource-specific quick actions on the last breadcrumb segment (e.g., "Open in new tab", "Copy DMR address", "View in model browser")

Capabilities

• Custom segment label — provide a function that maps AddressTemplate.Segment to a display string
• Extra actions — add action buttons/menu items to the breadcrumb's last segment

Registration

A registry keyed by Environment + AddressTemplate. When ResourceBreadcrumb renders segments for a matching template, the registry provides the rendering customizations.

Open Questions

• Is the value of custom breadcrumb rendering high enough to justify the complexity? The current key=value format is consistent and unambiguous.
• Should friendly names be centralized (a mapping from segment keys to display names) rather than per-resource?
• How should this interact with the onSegmentClick callback already provided by ResourceBreadcrumb?