feat: add built-in self loop edge routing support

[adithyaakrishna]

Description:

Implements first-class self-loop support in ng-diagram

What changed:

• Added new built-in self-loop routing
• Added self-loop metadata in routing context (selfLoopIndex, selfLoopCount)
• Added edgeRouting.selfLoop config + defaults
• Updated edge routing flow to detect and route source === target edges correctly
• Updated linking validation to allow valid same-node self-connections
• Added/updated tests and demo self-loop examples

[adithyaakrishna]

We use ng-diagram in prod and this feature is smth I built for our use-case, hence thought to contribute it to upstream as well :)

PS. Pls do lmk incase yall need any changes to be done, would be glad to do it

[lukasz-jazwa]

Hello @adithyaakrishna , thank you for your feature idea. It's great to hear that you use ng-diagram in prod.

Currently we're busy with 2 updates we're going to add to the library - bug fixes bundle + orgchart application template. Both will be available next week, and once we release them, we'll come back to your PR and evaluate it

[adithyaakrishna]

@lukasz-jazwa sounds good, thansk for the update :)