v0.2.0 — policy_slug on start_purchase + list_public_policies

Mirrors the TS SDK 0.2.0 changes (cf c3a57a0 in keysat-client-ts) so
all four language clients have parity on the tiered-purchase surface.

Breaking change on start_purchase: positional `(buyer_email,
redirect_url)` args replaced with a `&StartPurchaseOptions` struct.
Migration is mechanical:

  // before
  client.start_purchase(slug, None, None).await?;
  // after
  client.start_purchase(slug, &Default::default()).await?;

  // tier-aware
  client.start_purchase(slug, &StartPurchaseOptions {
      policy_slug: Some("pro"),
      buyer_email: Some("buyer@example.com"),
      ..Default::default()
  }).await?;

The struct has fields for buyer_email, buyer_note, redirect_url,
code, and the new policy_slug. New `list_public_policies` method
fetches the buyer-visible tier list (no auth) so an in-app tier
picker can render dynamically.

Lib + tests build clean; the example's anyhow-not-in-deps issue is
pre-existing and unrelated.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "keysat-licensing-client"
-version = "0.1.0"
+version = "0.2.0"
 edition = "2021"
 rust-version = "1.75"
 description = "Client library for Keysat. Verifies signed license keys offline and wraps the HTTP API for purchase and revocation checks."