Introduce Extensible Data Model (XDM) infrastructure

[shah]

Introduce the Object Model Builder (OMB) for XDM, a browser-native, framework-free utility that transforms DOM markup under a custom element into a structured, typed JavaScript object model.

OMB will treat HTML/XML markup as a declarative DSL and produce a pure JSON-compatible object graph with clear separation between physical syntax, structure, and typed semantic data.

See README for details.

TODO:

• auto-generate signals reactive state management mechanism (either using omb:signals or something similar)
• auto-generate SSE two-way server/client interaction mechanism (either using omb:sse or something similar)
• implement omb:interpolate strategy
• add <omb:include src="...">
• add <omb:value-source name="...">...JS function...</omb:value-source> similar to NEFS
• implement all legacy NEFS xdm:* features