index/db/commands/server: TTL indexes

createIndex({expireAt: 1}, {expireAfterSeconds: 60}) now deletes a
document once its indexed date is that many seconds old.

index.zig carries the option: Index.ttl (?i64), parsed from
expireAfterSeconds (int32/int64/integral double, within MongoDB's
[0, 2147483647]; 0 means "expire at the stored instant"), emitted by
spec_pairs and so persisted through the log and reported by listIndexes,
and compared by spec_equal — a same-name re-create with a different
expiry stays IndexOptionsConflict, as in MongoDB, since collMod does not
exist here. The bound keeps the value an int32 on the wire and makes the
emission cast unconditional. TTL is single-field only (a compound key is
error.TtlOnCompoundIndex); an absent option parses to null, so every
existing log record reparses unchanged.

db.zig sweeps: Engine.ttl_sweep(now_ms) walks each TTL index's entries
and deletes through the ordinary remove path, so an expiry is logged and
fsynced like any other write and holds across a restart. Expired ids are
duped before removal — remove frees the docs-map key that Entry.id
aliases — then sorted and deduped, because one document can be expired
by several entries (an array of dates expires on its earliest member,
which multikey expansion gives for free) or by several TTL indexes.
Selecting entries is a type test rather than a range lookup: bson
compare order ranks datetime above null, numbers and strings, so a
datetime upper bound would also select every value of a lesser type. A
sweep that deleted something checks the compaction threshold, since a
TTL-only workload never reaches the one in upsert.

commands.zig maps the new spec errors: CannotCreateIndex (67) for a
compound key or an out-of-range expiry, and InvalidIndexSpecificationOption
(197) for an expiry on {_id: 1}, which the idempotent _id no-op would
otherwise swallow.

server.zig runs the monitor as a member of the connection group, so the
existing group.cancel tears it down; it sleeps first, then sweeps under
the write lock, and logs rather than dies on a sweep failure.
--ttl-sweep-secs sets the interval (default 60, 0 leaves it unspawned).

Expiry is coarse by design, as in MongoDB: a document stays visible
until the next sweep, and a non-date value at the indexed path never
expires. Unit tests cover the spec round-trip and every rejection, the
sweep (inclusive cutoff, string/missing/future values untouched, second
sweep a no-op) and its survival of a reopen, and two TTL indexes over
one collection. e2e4.js exercises it through the Node driver.

README.md

@@ -39,8 +39,9 @@ mongosh --port 27017
   classes, groups, alternation, `i`/`s` options) `$not` `$and` `$or` `$nor`
   `$size` `$all` `$elemMatch`, with dot paths and array multikey semantics.
 - **Secondary indexes**: `createIndex`/`listIndexes`/`dropIndex` via the
-  three driver commands, single-field and compound, with `unique` and
-  `sparse` options, persisted in the log and rebuilt on open (compaction
+  three driver commands, single-field and compound, with `unique`,
+  `sparse` and `expireAfterSeconds` (TTL) options, persisted in the log
+  and rebuilt on open (compaction
   re-emits them). The query planner turns equality / `$in` / range
   predicates into index lookups across `find`, `count`, `update`,
   `delete`, `findAndModify`, and a leading `$match` in `aggregate`; every
@@ -68,13 +69,13 @@ src/
   bson.zig     BSON parse/serialize, ObjectId, canonical comparison order
   wire.zig     OP_MSG/OP_QUERY framing, message + reply builders
   commands.zig command dispatch (hello, CRUD, aggregate, admin, indexes)
-  server.zig   TCP accept loop, per-connection handlers
+  server.zig   TCP accept loop, per-connection handlers, TTL sweep monitor
   db.zig       in-memory engine: db → collection → _id → document maps
   storage.zig  append-only log: records, replay, CRC validation
   query.zig    filter matcher, regex engine, sort, projection
   index.zig    secondary indexes: entries, search, query planner
   update.zig   update operators with dot-path navigation
-  main.zig     CLI: --port, --bind, --db
+  main.zig     CLI: --port, --bind, --db, --ttl-sweep-secs
 ```
 
 ## Indexes
@@ -91,6 +92,18 @@ the query planner to narrow scans.
 - **Options**: `unique` (a conflicting write fails with E11000 naming the
   index; per-document entries are deduped first, so `{a: [1,1]}` is legal)
   and `sparse` (documents missing an indexed field are skipped).
+- **TTL**: `createIndex({expireAt: 1}, {expireAfterSeconds: 60})` deletes a
+  document once its indexed date is that many seconds old. A background
+  sweeper runs every `--ttl-sweep-secs` seconds (default 60, `0` disables
+  it) and deletes through the ordinary write path, so each expiry is logged
+  and fsynced and holds across a restart. As in MongoDB the option is
+  single-field only (a compound key is `CannotCreateIndex`, code 67),
+  `expireAfterSeconds` must be a whole number in `[0, 2147483647]` (`0`
+  means "expire at the stored instant"), a non-date value at the path never
+  expires, an array of dates expires on its earliest member, and expiry is
+  coarse: a document stays visible until the next sweep. Changing the
+  expiry of an existing index is `IndexOptionsConflict` — `collMod` is not
+  implemented.
 - **Multikey**: an array at an indexed path is indexed as a whole *and*
   element-wise, mirroring the query matcher exactly, so both
   `{tags: "a"}` and `{tags: ["a","b"]}` hit the index. A compound index
@@ -106,9 +119,9 @@ the query planner to narrow scans.
   serialization-ambiguous (int32 1, int64 1, double 1.0 compare equal but
   hash differently — those fall back to a scan, as do string/symbol/code).
 
-v1 limits: no index-accelerated sort, no hashed/text/geo/TTL/partial
-indexes, and entry insert/removal is O(n) (a sorted array) — fine for a
-light database, with a B-tree or id→entry map as the follow-up.
+v1 limits: no index-accelerated sort, no hashed/text/geo/partial indexes,
+and entry insert/removal is O(n) (a sorted array) — fine for a light
+database, with a B-tree or id→entry map as the follow-up.
 
 ## Not (yet) implemented