feat(concurrency): configurable cooldown + adaptive-headroom auto-reduce banner

Consumes the backend's concurrency-fixes (commit 2d27666) — additive to
[email protected], NO version bump. ConcurrencyStatusEntry gains
cooldownMs / autoReduced / autoReducedFrom? / notice?; new
ConcurrencyCooldownResponse / SetConcurrencyCooldownRequest + GET/PUT
/concurrency/cooldown/:providerId.

FE:
- Pure core (logic/view-model.ts): DEFAULT_COOLDOWN_MS; parseCooldownInput
  (non-negative int — 0 is valid, unlike the limit); normalizeCooldown;
  cooldownLabel ("350ms"/"1.2s"/"0ms (off)"); viewConcurrencyStatus extended
  (cooldown + autoReduce fields; auto-reduce → warning badge, not busy);
  viewAutoReduce/autoReduceNotices (banner view — prefers backend notice,
  synthesizes a fallback); summarizeStatus "N auto-reduced" fragment;
  normalizeConcurrencyStatus coerces new fields (immutable readonly build;
  autoReducedFrom/notice only when autoReduced===true); normalizeConcurrencyCooldown.
- Types (logic/types.ts): re-export the 2 new contract types +
  ConcurrencyCooldownResult + Get/SaveConcurrencyCooldown ports.
- UI: ConcurrencyCooldownRow.svelte (inline-edit cooldown + Save → PUT, seeded
  via the ChatLimitField pattern); AutoReduceBanner.svelte (dismissible banner —
  backend notice + "Was N, now M." + "Restore to N"); ConcurrencyView renders
  the cooldown per status card + the banner section. The banner persists while
  autoReduced===true, is dismissible, and clears automatically once a poll shows
  autoReduced===false after a restore PUT.
- Store (store.svelte.ts): getConcurrencyCooldown + setConcurrencyCooldown
  (surface 400/404/503 as ok:false; normalize at the seam) + interface decls.
- App.svelte: saveConcurrencyCooldown adapter → ConcurrencyView.
- Tests: +47 (view-model cooldown/auto-reduce/normalizers; component banner
  render, cooldown PUT, restore clears banner, dismiss; store cooldown GET/PUT).

Re-synced the file: dep (bun install) + re-mirrored .dispatch/transport-contract.
reference.md. typecheck 0/0, 1048 tests green (run twice), biome clean, build OK.

Worktree env: an untracked dispatch-backend → backend symlink was created in the
worktree parent so the canonical file:../dispatch-backend/... paths resolve
(NOT committed — per the §2d/§2j worktree convention).

1 files changed, 81 insertions, 0 deletions

diff --git a/src/features/concurrency/ui/AutoReduceBanner.svelte b/src/features/concurrency/ui/AutoReduceBanner.svelte
new file mode 100644
index 0000000..132ebc7
--- /dev/null
+++ b/src/features/concurrency/ui/AutoReduceBanner.svelte
@@ -0,0 +1,81 @@
+<script lang="ts">
+  import type { AutoReduceNotice } from "../logic/view-model";
+  import type { RestoreOutcome } from "../logic/types";
+
+  let {
+    notice,
+    onRestore,
+    onDismiss,
+  }: {
+    /** The auto-reduce banner view (providerId + message + from/current limit). */
+    notice: AutoReduceNotice;
+    /**
+     * "Restore to N" — PUT the limit back to `fromLimit`. Returns the outcome so
+     * a FAILED restore surfaces an inline error here (the banner owns its error
+     * display; the parent only refreshes on success).
+     */
+    onRestore: (providerId: string, limit: number) => Promise<RestoreOutcome>;
+    /** Hide this banner locally (persists hidden while autoReduced stays true). */
+    onDismiss: (providerId: string) => void;
+  } = $props();
+
+  let restoring = $state(false);
+  /** Inline restore error (e.g. "Concurrency service not available"); cleared on retry. */
+  let error = $state<string | null>(null);
+
+  async function handleRestore(): Promise<void> {
+    restoring = true;
+    error = null;
+    // The parent PUTs the limit + refreshes status on success; the banner clears
+    // once the next poll shows autoReduced===false. On failure the outcome is
+    // bubbled back here so the error shows inline next to the button.
+    const result = await onRestore(notice.providerId, notice.fromLimit);
+    restoring = false;
+    if (!result.ok) {
+      error = result.error;
+    }
+  }
+</script>
+
+<div
+  class="alert alert-warning flex flex-col gap-2 py-2 text-xs"
+  role="status"
+  data-testid={`auto-reduce-banner-${notice.providerId}`}
+>
+  <div class="flex items-start gap-2">
+    <span class="shrink-0">⚠</span>
+    <div class="flex-1">
+      <p>{notice.message}</p>
+      <p class="opacity-70">
+        Was {notice.fromLimit}, now {notice.currentLimit}.
+      </p>
+    </div>
+    <div class="flex shrink-0 items-center gap-1">
+      <!-- The "Restore to N" text stays visible while loading (only the spinner is
+           prepended) so the button keeps its accessible name during the PUT — a
+           spinner-only button loses its name for screen-reader users. -->
+      <button
+        type="button"
+        class="btn btn-warning btn-xs gap-1"
+        disabled={restoring}
+        onclick={handleRestore}
+      >
+        {#if restoring}
+          <span class="loading loading-spinner loading-xs"></span>
+        {/if}
+        Restore to {notice.fromLimit}
+      </button>
+      <button
+        type="button"
+        class="btn btn-ghost btn-xs"
+        aria-label={`Dismiss auto-reduce notice for ${notice.providerId}`}
+        onclick={() => onDismiss(notice.providerId)}
+      >
+        ✕
+      </button>
+    </div>
+  </div>
+  {#if error}
+    <p class="font-mono text-error" data-testid={`restore-error-${notice.providerId}`}>{error}</p>
+  {/if}
+</div>