Add type annotation and compound unit docs to dim-si README

[dyreby]

Add two sections to the dim-si README:

Type Annotations

Shows how to annotate function parameters and return types using Quantity<D> wrapper with dimension types like Length, Time, Velocity.

Compound Units

Shows how to create units like km/h by combining scale factors from existing units.

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Closes #7

[dyreby]

Note: This was drafted by my agent. If anything here is annoying or seems off, I'd appreciate an issue at https://github.com/dyreby/agent-stuff so I can improve it.

[gdtroszak]

The Type Annotations example imports Quantity from @isentropic/dim-quantity, but users of dim-si shouldn't need to reach into that package directly. dim-si should re-export the types they need.

Specifically, Linear and Affine (from dim-unit) are the right types for function annotations — they express whether a parameter is a regular quantity or an affine one (like absolute temperature). Quantity itself isn't needed since Linear and Affine cover both cases.

Proposed changes:

1. Re-export Linear and Affine from @isentropic/dim-si (root module)
2. Update the Type Annotations example to use Linear:

import type { Linear } from "@isentropic/dim-si";
import type { Length } from "@isentropic/dim-si/length";
import type { Time } from "@isentropic/dim-si/time";
import type { Velocity } from "@isentropic/dim-si/velocity";
import { divide } from "@isentropic/dim-si/ops";

function speed(
  distance: Linear<Length>,
  duration: Linear<Time>,
): Linear<Velocity> {
  return divide(distance, duration);
}

3. Add an Affine example to show the distinction:

import type { Affine } from "@isentropic/dim-si";
import type { Temperature } from "@isentropic/dim-si/temperature";
import { kelvin } from "@isentropic/dim-si/temperature";
import { q } from "@isentropic/dim-si/ops";

function toKelvin(temp: Affine<Temperature>): number {
  return q(temp).in(kelvin);
}

4. Update the prose to explain Linear vs Affine instead of referencing Quantity.

[gdtroszak]

This feels like it could live in the Quick Start rather than its own section — it's a natural next step after using built-in units.

[gdtroszak]

Closing this for now. I need to think more about what types to expose before providing guidance on type annotations and compound units. Will revisit in #7.