ci: shard the weekly mutation runs; no viewer job; six import mutants caught

The first scheduled run (2026-10-19) failed four ways: the viewer's
mutation job called the recipe retired in P2 slice 1 (owner, 2026-10-18:
no mutation runs on the GUI); the core and MCP runs were stopped at
GitHub's six hours; the import run missed six mutants.

- Every mutation recipe takes a shard (k/n, default all of it); the
  workflow runs the core in 8 shards, the store in 4, the importers in 2,
  the MCP server in 16, each its own job with a 350-minute limit.
- Test timeouts are three times each run's own baseline, never under the
  recipe's old fixed value, so slower runners don't report false
  timeouts.
- The mutants-gui job is gone.
- ledger-import: rule_marks_match_their_notes,
  asset_register_xlsx_headings_and_totals_rule and
  xlsx_missing_amounts_are_cells_only_on_ruled_rows catch the six
  (the tax-basis rule marks, a heading's alignment, the totals row's
  blank cells, and a blank amount cell only on ruled rows), each checked
  against its mutant by hand.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

.github/workflows/scheduled.yml

@@ -1,4 +1,8 @@
 # Weekly, and on demand: the slow layers (docs/testing.md).
+#
+# Mutation testing is split into shards (`just mutants-* k/n`), each its
+# own job, so every job ends well inside GitHub's six hours. The viewer has
+# no mutation job (owner, 2026-10-18).
 name: scheduled
 on:
   schedule:
@@ -29,54 +33,77 @@ jobs:
       - run: just fuzz ${{ matrix.target }} 600
   mutants:
     runs-on: ubuntu-24.04
+    timeout-minutes: 350
+    strategy:
+      fail-fast: false
+      matrix:
+        shard: [0, 1, 2, 3, 4, 5, 6, 7]
     steps:
       - uses: actions/checkout@v4
       - uses: jdx/mise-action@v2
-      - run: just mutants
-      - run: just mutants-store
+      - run: just mutants ${{ matrix.shard }}/8
       - if: always()
         uses: actions/upload-artifact@v4
         with:
-          name: mutants
-          path: |
-            mutants.out
-            mutants-store/mutants.out
-  mutants-import:
+          name: mutants-core-${{ matrix.shard }}
+          path: mutants.out
+  mutants-store:
     runs-on: ubuntu-24.04
+    timeout-minutes: 350
+    strategy:
+      fail-fast: false
+      matrix:
+        shard: [0, 1, 2, 3]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: jdx/mise-action@v2
+      - run: just mutants-store ${{ matrix.shard }}/4
+      - if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: mutants-store-${{ matrix.shard }}
+          path: mutants-store/mutants.out
+  mutants-import:
+    runs-on: ubuntu-24.04
+    timeout-minutes: 350
+    strategy:
+      fail-fast: false
+      matrix:
+        shard: [0, 1]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: jdx/mise-action@v2
+      - run: just mutants-import ${{ matrix.shard }}/2
+      - if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: mutants-import-${{ matrix.shard }}
+          path: mutants-import/mutants.out
+  mutants-config:
+    runs-on: ubuntu-24.04
+    timeout-minutes: 350
     steps:
       - uses: actions/checkout@v4
       - uses: jdx/mise-action@v2
-      - run: just mutants-import
       - run: just mutants-config
       - if: always()
         uses: actions/upload-artifact@v4
         with:
-          name: mutants-import
-          path: |
-            mutants-import/mutants.out
-            mutants-config/mutants.out
+          name: mutants-config
+          path: mutants-config/mutants.out
   mutants-mcp:
     runs-on: ubuntu-24.04
     timeout-minutes: 350
+    strategy:
+      fail-fast: false
+      matrix:
+        shard: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
     steps:
       - uses: actions/checkout@v4
       - uses: jdx/mise-action@v2
-      - run: just mutants-mcp
+      - run: just mutants-mcp ${{ matrix.shard }}/16
       - if: always()
         uses: actions/upload-artifact@v4
         with:
-          name: mutants-mcp
+          name: mutants-mcp-${{ matrix.shard }}
           path: mutants-mcp/mutants.out
-  mutants-gui:
-    runs-on: ubuntu-24.04
-    timeout-minutes: 350
-    steps:
-      - uses: actions/checkout@v4
-      - uses: jdx/mise-action@v2
-      - run: sudo apt-get update && sudo apt-get install -y libxkbcommon-dev libgtk-3-dev
-      - run: just mutants-gui
-      - if: always()
-        uses: actions/upload-artifact@v4
-        with:
-          name: mutants-gui
-          path: mutants-gui/mutants.out