feat(viewer): reload/conflict reducer and editability rules

- viewerState.ts: pure reducer for the clean/dirty, same/changed/gone,
  container-down and overwrite-on-save states (spec §5), plus the
  pollEffect/canSave helpers EditorPane will drive off.
- editability.ts: classifies a fetched file as text/image/binary and
  decides whether it is editable, deferring to Rust's readonly_reason
  when it refuses.

Per preflight P1, languages.ts/.test.ts move to Task 10 (needs the
CodeMirror packages Task 6 installs; out of scope for this task's
worktree). Per P3, the "reloaded" action now carries `truncated` and
`polledHash` so a poll-driven reload of a truncated (prefix-hash-only)
file adopts the polled full-file hash instead of re-triggering a
reload on every subsequent poll -- with a reducer test covering it.
Per P13, tightened the poll_failed/canSave test to start from a dirty
doc so it actually exercises containerDown rather than passing only
because the doc was clean.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

app/src/viewer/editability.ts

@@ -0,0 +1,15 @@
+import { imageMimeFor, looksBinary, TEXT_PREVIEW_LIMIT } from "../components/projects/home/filePreview";
+import type { ViewerFile } from "../lib/types";
+
+export type ViewerKind = "text" | "image" | "binary";
+export interface Editability { kind: ViewerKind; editable: boolean; reason: string | null }
+
+const MIB = TEXT_PREVIEW_LIMIT / (1024 * 1024);
+
+export function classifyViewerFile(path: string, file: ViewerFile, bytes: Uint8Array): Editability {
+  if (imageMimeFor(path)) return { kind: "image", editable: false, reason: "Images are shown, not edited." };
+  if (looksBinary(bytes)) return { kind: "binary", editable: false, reason: "This file is not text." };
+  if (file.truncated) return { kind: "text", editable: false, reason: `Files over ${MIB} MiB are read-only.` };
+  if (!file.editable) return { kind: "text", editable: false, reason: file.readonly_reason ?? "This location is read-only." };
+  return { kind: "text", editable: true, reason: null };
+}