LMDBDict now subtypes AbstractDict{K,V}, so merge!/filter!/pairs/==/
hash/lazy keys/lazy values come from the standard library. The
hand-rolled keys/values/collect overloads with a `prefix` kwarg are
gone; `keys(d)` and `values(d)` are the lazy `KeySet`/`ValueIterator`
the AbstractDict contract promises.

Iteration follows LLVM.jl's `BasicBlockInstructionSet` pattern: the
state is `(txn, cur)`, opened on the first `iterate` call (analogous
to LLVM's `LLVMGetFirstInstruction` seed) and advanced with
`MDB_NEXT` each step. Normal completion commits the txn and closes
the cursor; early `break`/throw is reclaimed by the Step-9 finalizers.

Other AbstractDict-shaped changes:

  - `getindex(d, k)` throws `KeyError(k)` on miss (was `LMDBError`).
  - `pop!(d, k)` throws `KeyError`; `pop!(d, k, default)` returns the
    default; both are atomic in one write txn.
  - `length(d)` via `mdb_stat`; `isempty`, `empty!` follow.
  - `delete!(d, k)` no longer throws on missing keys (Base contract).

Prefix-scoped scans move to LMDB-namespaced helpers — `LMDB.scan(d)`,
`LMDB.scan_keys(d)`, `LMDB.scan_values(d)` — alongside `list_dirs`
and `valuesize`.

Defaults:

  - `MDB_NOTLS` is now set on the LMDBDict env, matching py-lmdb. Without
    it, a `length(d)` call mid-iteration tripped MDB_BAD_RSLOT.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

…sor.

cur.jl loses ~100 lines of legacy iterator scaffolding that nothing in
the new API exercises:

  - LMDBIterator{R} + ReturnKeys/ReturnValues/ReturnBoth/ReturnValueSize
  - DirectoryLister + process_returns + init_values + arcopy
  - keys(cur, T; prefix) / Base.values(cur, T; prefix) / a broken
    iterate(cur, K, V) that referenced an undefined `prefix` variable
  - get(cur, key, T, op) — superseded by seek!+value at the right op

`mbd_unpack` was always a typo for `mdb_unpack`; renamed in src and tests.

Cursor now stores its parent DBI alongside its parent Transaction;
database(cur) returns it directly instead of round-tripping through
`mdb_cursor_dbi` and synthesizing a fresh wrapper with an empty name.

Adds the AbstractDict gap that Base.Dict supplies but
Base.AbstractDict does not — `pop!(d::LMDBDict)` (no key form) pops
the lexicographically-first entry, throws ArgumentError if empty.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

AbstractDict's default merge! / mergewith! / filter! call d[k]=v or
delete!(d, k) in a loop, and each one of those opens its own LMDB write
txn (with its own commit/fsync). For a 1k-entry merge! that's 1k
transactions, which dominates wall time on durable storage. Override
the three to land in one txn instead.