feat(snippets): record merge provenance on the survivor

Merge kept the target's fields, unioned locations and tags, and trashed
the sources — recording nothing about what it absorbed. If a variant
handled an edge case the survivor doesn't, that difference left the
visible record entirely; recovering it meant knowing to go digging in
the trash.

The survivor now carries `merged_from`: a "**Merged from:** #2, #3" line
in the body for humans, and the same list in the `data` mirror for
queries, written from one value like every other field (#2087).

It accumulates rather than replaces — a target merged twice keeps both
histories — and skipped sources (cross-owner, per #231) are excluded, so
the record never claims to contain something it never absorbed.

Ordinary edits carry it forward. update_snippet recomposes body and
mirror from scratch, so an omission there would silently erase the
history on the next unrelated edit; that path is pinned by its own test,
including the pre-0070 case where the body line is the only copy.

Surfaced in the snippet detail view as a "Merged from" row — the view
renders parsed fields, not the raw body, so the body line alone would
have been invisible to the operator (rule #27).

Un-merge, the other half of #2087, stays open: restoring a source from
trash still doesn't strip its locations off the survivor, and what
partial un-merge should mean is a design question, not a coding one.
`merged_from` is the record that makes it tractable.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLwAaV4DQEmVyn496HnEvt

frontend/src/views/SnippetDetailView.vue

@@ -23,6 +23,10 @@ const canWrite = computed(() => {
 
 const locations = computed(() => snippet.value?.snippet.locations ?? []);
 
+// What this record absorbed. Merge keeps the target's fields and trashes the
+// sources, so this line is the only visible trace that the variants existed.
+const mergedFrom = computed(() => snippet.value?.snippet.merged_from ?? []);
+
 function locParts(loc: { repo: string; path: string; symbol: string }): string[] {
   return [loc.repo, loc.path, loc.symbol].filter((p) => p && p.trim());
 }
@@ -110,6 +114,15 @@ async function confirmDelete() {
           <dt>Language</dt>
           <dd>{{ snippet.snippet.language }}</dd>
         </template>
+        <template v-if="mergedFrom.length">
+          <dt>Merged from</dt>
+          <dd class="merged-from">
+            <span v-for="mid in mergedFrom" :key="mid">#{{ mid }}</span>
+            <span class="merged-hint">
+              folded in here — the originals are in the trash
+            </span>
+          </dd>
+        </template>
       </dl>
 
       <div class="code-block">
@@ -274,6 +287,16 @@ async function confirmDelete() {
   gap: 0.35rem;
   align-items: center;
 }
+.merged-from {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.35rem;
+  align-items: baseline;
+}
+.merged-hint {
+  color: var(--color-text-muted);
+  font-size: 0.8rem;
+}
 
 .code-block {
   border: 1px solid var(--color-border);