feat: add /p/pflow/metamodel package and /r/stackdump/home realm

[stackdump]

This build adds my home realm: /r/stackdump/home which is also an example usage of the /p/pflow/metamodel package.

The primary goal of the package is to provide a useful state machine library that can be visualized using gno.land web.

This is accomplished by generating mardown links and images using base64 encoded image data:

[![pflow](https://pflow.dev/img/?b=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)](https://pflow.dev/?b=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)

This package uses base64 encoded data URLs to allow SVG images to be rendered.

I'd like to confirm that this behavior is considered good practice? or is there a better way to accomplish image rendering using gno?

Here's an example usage I'd expect from users of the package.

import (
	mm "gno.land/p/pflow/metamodel"
)

// define model
var m = exampleModel()

// Use m.ToLinkMarkdown() to embed the petri net in markdown
// ![Petri-net](https://pflow.dev/img/b=<BASE64DATA>
_ := m.ToLinkMarkdown()

// define petri net model using metamodel package
func exampleModel() *mm.Pflow {
	return &mm.Pflow{
		Places: map[string]mm.Place{
			"place0": {Offset: 0, Initial: 1, Capacity:[ 3](http://127.0.0.1:8888/r/stackdump/home#L3), X: 130, Y: 207},
		},
		Transitions: map[string]mm.Transition{
			"txn0": {X:[ 4](http://127.0.0.1:8888/r/stackdump/home#L4)6, Y: 116},
			"txn1": {X: 227, Y: 112},
			"txn2": {X: 43, Y: 307},
			"txn3": {X: 235, Y: 306},
		},
		Arrows: []mm.Arrow{
			{Source: "txn0", Target: "place0"},
			{Source: "place0", Target: "txn1", Weight: 3},
			{Source: "txn2", Target: "place0", Weight: 3, Inhibit: true},
			{Source: "place0", Target: "txn3", Inhibit: true},
		},
	}
}

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[stackdump]

I've overwritten this w/ a new version - now trying to satisfy the CI/CD requirements