Merge api/non-exhaustive-options (#74)

`ConvergenceReport` is only ever constructed by `converge` /
`converge_partial`, so marking it costs a caller nothing and makes a
future field additive.

`ConvergenceOptions` and `GameOptions` deliberately stay constructible,
against #74's recommendation, because trying it turned up a cost the
issue did not anticipate. `Default::default` is not a `const fn`, so
`#[non_exhaustive]` + `..Default::default()` — the pattern that makes
marking an options struct cheap — does not work in a `const`:

    error[E0639]: cannot create non-exhaustive struct using struct expression
      --> tests/competitor_config.rs:13:41
       |
    13 | const CONVERGENCE: ConvergenceOptions = ConvergenceOptions {

`ConvergenceOptions` is `Copy` and a natural const; there is no
workaround from outside the crate. That cost is permanent, and adding a
field is a one-time major bump. The reasoning is recorded on the type so
the next person does not rediscover it.

Refs #74.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hcFjNDmHXZF8URGLku5zZ

src/convergence.rs

@@ -11,6 +11,14 @@ use smallvec::SmallVec;
 /// carried by `GameOptions` for a single match scored without a history. The
 /// defaults are the crate's globals: [`ITERATIONS`](crate::ITERATIONS),
 /// [`EPSILON`](crate::EPSILON), and undamped EP.
+///
+/// Deliberately **not** `#[non_exhaustive]`, unlike [`ConvergenceReport`]. The
+/// usual argument for marking an options struct is that `..Default::default()`
+/// makes a future field additive — but `Default::default` is not a `const fn`,
+/// so marking it would make
+/// `const OPTS: ConvergenceOptions = ConvergenceOptions { .. }` impossible from
+/// outside the crate, with no workaround. This type is `Copy` and a natural
+/// const; that cost is permanent, and adding a field is a one-time major bump.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct ConvergenceOptions {
     /// Hard cap on full forward+backward sweeps.
@@ -89,7 +97,12 @@ impl Default for ConvergenceOptions {
 /// [`InferenceError::NotConverged`](crate::InferenceError::NotConverged) there.
 /// From [`History::converge_partial`](crate::History::converge_partial) it may
 /// not be, and `converged` is what says so.
+/// Constructed only by `converge` / `converge_partial`, never by a caller, so
+/// `#[non_exhaustive]` costs nothing here and lets a future field be additive.
+/// The two *options* structs deliberately do not carry it — see the note on
+/// [`ConvergenceOptions`].
 #[derive(Clone, Debug, PartialEq)]
+#[non_exhaustive]
 pub struct ConvergenceReport {
     /// Full forward+backward sweeps actually run. `0` for a history with no
     /// time slices, which is converged trivially.