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 the owner 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

fuzz/fuzz_targets/fuzz_journal_line.rs

@@ -1,6 +1,6 @@
 // Fuzz target: journal line parsing must never panic, and the canonical
 // form must be a fixed point: parse(canonical(x)) re-serializes to the
-// same canonical bytes. Staged here; `cargo fuzz init` wires it up.
+// same canonical bytes.
 #![no_main]
 
 use libfuzzer_sys::fuzz_target;
@@ -8,10 +8,11 @@ use ledger_store::JournalLine;
 
 fuzz_target!(|data: &[u8]| {
     // Garbage must return Err, never panic.
-    if let Ok(line) = serde_json::from_slice::<JournalLine>(data) {
+    // Parse as the journal reader does (it refuses floats: the journal
+    // never holds one, and a float's canonical form isn't a fixed point).
+    if let Ok(line) = JournalLine::parse(data) {
         let canon = line.to_canonical();
-        let line2: JournalLine =
-            serde_json::from_str(&canon).expect("canonical form must re-parse");
+        let line2 = JournalLine::parse(canon.as_bytes()).expect("canonical form must re-parse");
         assert_eq!(
             line2.to_canonical(),
             canon,