milestone 26: upstream health answers for the selected period

src/upstream/health.zig

@@ -45,6 +45,12 @@ pub const Config = struct {
 /// `State.window`.
 pub const window_len = 32;
 
+/// Bytes kept of an `@errorName`, truncated to fit. Shared rather than repeated:
+/// `history.Accumulator.Cell` and `upstream_history_repo.WindowStats` carry the
+/// same name through the minute aggregates, and three buffers of three different
+/// sizes would truncate one error name three ways.
+pub const error_name_capacity = 48;
+
 /// The shift is capped so `base_backoff_ms << shift` cannot run away; by then
 /// `max_backoff_ms` has clamped the result many doublings ago.
 const max_shift = 20;
@@ -55,7 +61,7 @@ pub const State = struct {
     total_failures: u64,
     last_success_at: ?std.Io.Timestamp,
     last_error_at: ?std.Io.Timestamp,
-    last_error_buf: [48]u8,
+    last_error_buf: [error_name_capacity]u8,
     /// Length of the `@errorName` held in `last_error_buf`, truncated to fit.
     last_error_len: u8,
     backoff_until: ?std.Io.Timestamp,