query: support bare regex filter values and array-index dot paths

The Node driver sends {field: /re/} as a BSON regex element (type 0x0B),
which the matcher previously only handled via the $regex operator form;
and dot paths with numeric segments (tags.0) were ignored because array
descent only recursed into embedded docs. Both are part of standard
MongoDB query semantics and were caught by the driver e2e suite.

server: unbounded Io async limit so the accept loop never wedges, and
treat header-read failures (client RST on pool teardown) as clean
disconnects. With the default cpu_count-1 limit, groupAsync's eager
fallback ran connection handlers inline on the accept-loop fiber once
that many connections were alive, stalling accept() and timing out
handshakes for further clients.

Add tests/e2e/: official driver CRUD, concurrency, and kill -9 recovery
suites (29 + 2 + 3 checks), plus unit tests for the query fixes.

tests/e2e/README.md

@@ -0,0 +1,29 @@
+# End-to-end tests with the official MongoDB Node.js driver
+
+These exercise mongo-light from a real driver over TCP: full CRUD, query
+operators, aggregation, error codes, concurrent clients, and crash recovery.
+
+## Setup
+
+```sh
+cd tests/e2e
+npm init -y >/dev/null
+npm install mongodb
+```
+
+## Run
+
+Start the server, then run the suites against it (defaults to port 27020):
+
+```sh
+zig build
+zig-out/bin/mongo-light --port 27020 --db /tmp/ml-e2e.log &
+
+node tests/e2e/e2e.js          # CRUD + operators + aggregate + errors (29 checks)
+node tests/e2e/e2e2.js concurrent   # 8 clients: 4 writers + 4 readers (2 checks)
+node tests/e2e/e2e2.js crash-a      # write 50 docs, then kill -9 the server
+node tests/e2e/e2e2.js crash-b      # restart and verify all 50 survived
+```
+
+`e2e2.js concurrent` is safe to repeat against a running server (it drops its
+collection first); `crash-a`/`crash-b` are two halves of one scenario.