terminal/snapshot: report invalid decoder states

Decoder.next treated calls before READY and calls after any prior decode
error as unreachable. Network or mux glue that retried after a truncated
history record, or invoked next before setup completed, could therefore
turn a recoverable protocol misuse into a process panic.

Add DecoderNotReady and DecoderFailed to NextError and return them for
the start and failed states. Keep finished calls idempotent, and cover
both an early call and a retry after FINISH truncation.

src/terminal/snapshot/snapshot.zig

@@ -413,6 +413,12 @@ pub const Decoder = struct {
         page.DecodeError ||
         Allocator.Error ||
         error{
+            /// `next` was called before `ready` completed successfully.
+            DecoderNotReady,
+
+            /// A prior `next` call failed and invalidated the stream position.
+            DecoderFailed,
+
             /// A HISTORY names a key not declared by TERMINAL.
             UnexpectedHistoryKey,
 
@@ -457,7 +463,8 @@ pub const Decoder = struct {
         switch (self.state) {
             // `ready` must succeed before history exists to decode, and a
             // failed decoder no longer knows its stream position.
-            .start, .failed => unreachable,
+            .start => return error.DecoderNotReady,
+            .failed => return error.DecoderFailed,
             .finished => return null,
             .history => {},
         }
@@ -1774,6 +1781,23 @@ test "incremental decode restores a renderable terminal at READY" {
     try testing.expectEqualStrings(&test_complete_fixture, reencoded.written());
 }
 
+test "incremental next reports invalid decoder states" {
+    const testing = std.testing;
+    var source: std.Io.Reader = .fixed("");
+    var decoder: Decoder = .init(&source);
+    var terminal_value = try Terminal.init(
+        testing.io,
+        testing.allocator,
+        .{ .cols = 1, .rows = 1 },
+    );
+    defer terminal_value.deinit(testing.allocator);
+
+    try testing.expectError(
+        error.DecoderNotReady,
+        decoder.next(testing.allocator, &terminal_value),
+    );
+}
+
 test "incremental decode applies history below live PTY output" {
     const testing = std.testing;
 
@@ -2065,6 +2089,10 @@ test "incremental decode failure leaves applied history usable" {
         error.EndOfStream,
         decoder.next(testing.allocator, &restored),
     );
+    try testing.expectError(
+        error.DecoderFailed,
+        decoder.next(testing.allocator, &restored),
+    );
 
     // The transferred terminal was never owned by the decoder: it remains
     // valid with the contiguous history prefix that did apply, and only