feat(core): attribute-copy engine (plan C7)

- planCopy previews the changes and the skipped items for one target; applyCopy
  applies exactly the planned changes.
- Default selection copies only the coping width; coping length and pile length
  are off by default (spec §9). Pile items are skipped when the foundation type
  differs. Coordinates, IPs, bearings and elevations are not in any item.
- Equal values are not counted, so the preview count equals the applied count.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

src/core/copy.ts

@@ -0,0 +1,62 @@
+import type { EditableKey } from './changes';
+import type { PierInput } from './types';
+
+/**
+ * 속성 복사 (spec §9 · plan C7). Copies chosen attributes from one pier to others.
+ * Values the spec never copies (coordinates, IP, elevations, names) are not in any item, so they cannot be copied.
+ */
+export type CopyItem = 'coping-width' | 'coping-length' | 'pile-length';
+
+interface ItemRule {
+  label: string;
+  keys: EditableKey[];
+  /** Off by default (spec: 말뚝 길이·코핑 길이 기본 해제). */
+  defaultOn: boolean;
+  /** Pile items only apply when the foundation type matches (spec §9 건너뜀). */
+  pileItem: boolean;
+}
+
+export const COPY_ITEMS: Record<CopyItem, ItemRule> = {
+  'coping-width': { label: '코핑 폭', keys: ['copingWidthMm'], defaultOn: true, pileItem: false },
+  'coping-length': { label: '코핑 길이 L', keys: ['copingLengthMm'], defaultOn: false, pileItem: false },
+  'pile-length': { label: '말뚝 길이', keys: ['pileLengthMm'], defaultOn: false, pileItem: true },
+};
+
+export const DEFAULT_COPY_ITEMS: CopyItem[] = (Object.keys(COPY_ITEMS) as CopyItem[]).filter((k) => COPY_ITEMS[k].defaultOn);
+
+export interface CopyChange {
+  key: EditableKey;
+  from: number;
+  to: number;
+}
+
+export interface CopyPlan {
+  changes: CopyChange[];
+  /** Items not applied to this target, with the reason shown in the preview. */
+  skipped: { item: CopyItem; reason: string }[];
+}
+
+/** Preview for one target: what would change and what is skipped. Same values are not counted as changes. */
+export function planCopy(source: PierInput, target: PierInput, items: CopyItem[]): CopyPlan {
+  const changes: CopyChange[] = [];
+  const skipped: CopyPlan['skipped'] = [];
+  for (const item of items) {
+    const rule = COPY_ITEMS[item];
+    if (rule.pileItem && source.foundationType !== target.foundationType) {
+      skipped.push({ item, reason: `기초 형식이 다릅니다 (${target.foundationType})` });
+      continue;
+    }
+    for (const key of rule.keys) {
+      if (source[key] === target[key]) continue;
+      changes.push({ key, from: target[key], to: source[key] });
+    }
+  }
+  return { changes, skipped };
+}
+
+/** Applies a plan to a target input. Only the planned keys change. */
+export function applyCopy(target: PierInput, plan: CopyPlan): PierInput {
+  const next = { ...target };
+  for (const c of plan.changes) next[c.key] = c.to;
+  return next;
+}