plan: drop segfaults when dispatched in-process

Found while writing the positional-refusal tests and not caused by them --
it reproduces at 3c5eee2 with that work stashed. One insert followed by one
drop against a TestDb engine terminates with SIGSEGV and no stack trace,
while 25 insert/update/distinct/drop cycles over the wire are clean, so what
differs is the in-process caller rather than the command.

Worth more than it looks: D1's architecture is "in-process server now,
library + C API later", and this is that path. `drop` has no unit test in
the tree at all, which is why it went unseen.

PLAN.md

@@ -1045,6 +1045,19 @@ has to be its own commit with its own re-recorded scorecard.
   `$out`/`$merge` durability semantics, whether the expression evaluator is
   shared with M3's pipeline updates, and whether `allowDiskUse` has to stop
   being a lie.
+- **`drop` segfaults when dispatched in-process** — found while writing the
+  positional-refusal tests, unrelated to them, and **reproduces at `3c5eee2`
+  with the change stashed**, so it is not caused by that work. A single
+  `insert` followed by a single `drop` against a `TestDb` engine terminates
+  with SIGSEGV and no stack trace; dropping a collection that never existed is
+  fine. The wire path is clean — 25 insert / refused-update / distinct / drop
+  cycles against a live server on `:27020` all passed — so what differs is the
+  in-process caller, not the command. That matters more than it looks: D1's
+  whole architecture is "in-process server now, library + C API later", and
+  this is the embedding path. **`drop` has no unit test at all** — the only
+  `parse_fake_msg("drop", ...)` calls in the tree were the scratch ones written
+  to find this — which is why it went unseen. Needs its own commit: reproduce
+  it in a committed test first, then fix.
 - **M3 update operators** — open. `distinct` landed first because it was a
   whole missing command with no dependencies, and measuring it turned up three
   things worth keeping, none of which are `distinct`'s to fix: