fix(dedup): compare the artefact, not the prose describing it

The snippet gate was reading the wrong field, and #2464's UI recipes made it
measurable in both directions at once:

  .btn-danger vs .btn-danger-outline   0.92   siblings, BLOCKED
  .btn-primary re-recorded verbatim
  under a different name              <0.90   a literal copy, ALLOWED

The second is what settles it. Identical code at an identical repo·path·symbol
sailed through because the description differed, while two deliberately
parallel variants were refused because theirs did not. A snippet's embedded
document is mostly prose ABOUT the code, so no threshold fixes this: lowering
it blocks more siblings, raising it admits more copies.

So structure decides. Two exact signals, both index-served off the notes.data
mirror that already exists, no migration and no backfill:

  location  the same named thing in the same file. Requires BOTH path and
            symbol — a path alone is a directory of artefacts, and matching on
            it would refuse every second recipe from one stylesheet.
  code      byte-identical code anywhere, via the same fingerprint the drift
            check uses.

The semantic arm survives as a backstop for a genuine reword that shares
neither, raised to 0.96 so it sits above the 0.92 band where real variants
live. Structural hits say what they matched instead of hedging with "similar",
and point at merge rather than update — two records of one artefact is what
merge exists to fold back together.

find_duplicate_snippets gets the same correction: pairs where both snippets
name a symbol, name DIFFERENT symbols, and hold different code are variants,
not copies. Without it a design system's button family reports as one merge
set — eight recipes, every direct pair over the floor, top score 0.92, one
click from collapsing a component family. The cost is real and stated in the
code: a helper recorded twice under two names no longer reports. That trade
favours the report being usable, and same-symbol and unnamed duplicates — how
re-recording usually looks — still surface. The filter fails open, so a lookup
failure degrades to the old unfiltered report rather than to a reassuring
empty one.

resolve_locations extracted: compose_body, create_snippet and now the gate each
had their own copy of the repo/path/symbol shorthand fallback, and the gate is
the one where a disagreement would mean matching a location the record won't be
stored with. Applied to both create surfaces (#33) — the web UI must not be the
way to record what the agent was stopped from writing.

Refs #2518, #2464

src/scribe/mcp/tools/snippets.py

@@ -127,12 +127,19 @@ async def create_snippet(
         force: Bypass the near-duplicate gate (see below).
 
     Returns the created snippet (including a parsed `snippet` field), OR — when a
-    near-duplicate snippet already exists and force is false — {"duplicate": true,
+    duplicate already exists and force is false — {"duplicate": true,
     "existing_id": ..., "message": ...} and nothing is created. When that happens
     and it really is the same reusable thing found in another place, prefer
     merge_snippets(existing_id, [new...]) — or record then merge — to unify them
     into ONE canonical record (which then carries every call site as a location),
     rather than forcing a second copy with force=true.
+
+    WHAT THE GATE MATCHES ON. Exact identity first — an existing snippet at the
+    same repo · path · symbol, or holding byte-identical code. Those are certain,
+    and force is almost never the right answer to them. Only then a semantic
+    check, held to a high bar so that VARIANTS of one component are not refused:
+    `.btn-primary` and `.btn-secondary` read alike and are two different things,
+    so record both (#2518).
     """
     if not (name or "").strip() or not (code or "").strip():
         raise ValueError("create_snippet requires a non-empty name and code")
@@ -148,6 +155,10 @@ async def create_snippet(
         dup = await dedup_svc.find_duplicate_note(
             uid, title, body, project_id=project_id or None,
             is_task=False, note_type=snippets_svc.SNIPPET_NOTE_TYPE,
+            # The artefact itself, not just its description — the gate compares
+            # location and code before it compares prose (#2518).
+            code=code,
+            locations=snippets_svc.resolve_locations(repo, path, symbol, locations),
         )
         if dup is not None:
             return dedup_svc.duplicate_response(dup, "snippet")