[EXT] Detect and warn on prefix collisions between extensions

[flyingrobots]

Problem

Two extensions can both declare the same node prefix (e.g., both declare task:) with no warning or error. This creates silent ambiguity — the second registration's view definitions silently win.

Work

In registerExtension() in src/extension.js:

• Build an index of which extension owns which prefix
• On registration, check the incoming extension's domain.prefixes against already-registered prefixes
• If collision: throw (strict mode) or warn (lenient mode) with message identifying which extension already owns the prefix

Also: edge type collisions.

Ref: #252

[flyingrobots]

Resolved in 79fac6c (PR #270)