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.

README.md

@@ -51,7 +51,18 @@ The server enforces revocation live and does trust-on-first-use fingerprint bind
 ## Purchase flow
 
 ```rust
-let session = client.start_purchase("my-product", None, None).await?;
+use licensing_client::StartPurchaseOptions;
+
+// Default tier:
+let session = client.start_purchase("my-product", &Default::default()).await?;
+
+// Specific tier (e.g. Pro):
+let session = client.start_purchase("my-product", &StartPurchaseOptions {
+    policy_slug: Some("pro"),
+    buyer_email: Some("buyer@example.com"),
+    ..Default::default()
+}).await?;
+
 // open session.checkout_url in the user's browser
 loop {
     let poll = client.poll_purchase(&session.invoice_id).await?;