test: fuzzing, Miri, benchmarks, CI, and a strategy note; the journal refuses floats

Max-testing pass, ranks 4-6 and step 9 for ledger-core.

Fuzzing (rank 4): fuzz_op_sequence now drives balanced pairs, foreign
lines, periods, closes, and amounts from raw decimal bits, and asserts
the books stay valid (TB balances, posted entries validate, no draft in
a closed period); new fuzz_core_serde (every persisted core type
round-trips) and fuzz_store_open (a corrupt journal is an error, never a
panic); hangs count (-timeout=10). 54M executions; one crash:
- fuzz_journal_line: a number too big for an integer parsed as a float,
  and its canonical form (which the hash chain covers) re-canonicalized
  differently. The journal never holds floats (R-2), so the reader now
  refuses them (JournalLine::parse, used by every journal reader);
  regression a_journal_line_with_a_float_is_refused. Checked first: the
  real books' journal has none. Rerun: 13.2M executions, clean.
Seeds in fuzz/seeds/ (synthetic, minimal); the grown corpora stay out of
git. `just fuzz <target> <seconds>`.

Miri (rank 4): the core's unit, bridge, and invariant tests under Miri
(our code has no unsafe; rust_decimal and chrono do): no undefined
behavior. `just miri`. A pinned nightly (nightly-2026-10-01, with miri)
is now in mise.toml.

Benchmarks (rank 6): criterion (new dev-dependency: the playbook's Rust
benchmark tool; none existed) in benches/books.rs; baselines in
docs/testing.md. The store's per-command rehearsal clone costs ~1 ms at
10k entries. No budget yet, so nothing gates on them. `just bench`.

CI: .github/workflows/check.yml runs `just check` on every push;
scheduled.yml runs proofs, the GnuCash oracle, fuzzing, and mutation
weekly. Tools come from mise.toml.

docs/testing.md: every layer, its oracle rank, command, runtime, and
gate; benchmark baselines; the ASSUMED invariants for Stephen to reject;
the gap list with triggers. docs/oracle.md's fuzzing paragraph had
claimed persistent corpora that didn't exist; corrected.

Also: ledger-import now inherits the workspace lints (nightly cargo
warned).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

.github/workflows/check.yml

@@ -0,0 +1,15 @@
+# Every change: format, clippy and rustdoc with zero warnings, every test.
+# Tools come from mise.toml, the same as on a laptop.
+name: check
+on:
+  push:
+  pull_request:
+jobs:
+  check:
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: jdx/mise-action@v2
+      - name: GUI build dependencies
+        run: sudo apt-get update && sudo apt-get install -y libxkbcommon-dev libgtk-3-dev
+      - run: just check

.github/workflows/scheduled.yml

@@ -0,0 +1,40 @@
+# Weekly, and on demand: the slow layers (docs/testing.md).
+name: scheduled
+on:
+  schedule:
+    - cron: "17 6 * * 1"
+  workflow_dispatch:
+jobs:
+  prove:
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: jdx/mise-action@v2
+      - run: just prove
+  oracle:
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: jdx/mise-action@v2
+      - run: just oracle
+  fuzz:
+    runs-on: ubuntu-24.04
+    strategy:
+      fail-fast: false
+      matrix:
+        target: [fuzz_journal_line, fuzz_entry_validate, fuzz_op_sequence, fuzz_core_serde, fuzz_store_open]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: jdx/mise-action@v2
+      - run: just fuzz ${{ matrix.target }} 600
+  mutants:
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: jdx/mise-action@v2
+      - run: just mutants
+      - if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: mutants
+          path: mutants.out