plan: sessions, and the three measurements that corrected it

Records what `lsid` support is and what it deliberately is not, so M4 inherits
the decisions rather than the questions. The part worth keeping is not the
design but its corrections: three of the assumptions this stage was planned on
turned out to be wrong when measured against a real mongod, and one of them --
that unknown fields inside `lsid` are tolerated -- would have shipped a
divergence nothing in the test corpus could have caught.

Also states that the scorecard did not move, 194/97/196 either side, which was
the prediction rather than a surprise: the corpus has no session entities at
all. What is new is that the prediction is now checkable -- after the event
assertions landed, a command wrongly refused here would show up as a changed
event stream instead of silently.

PLAN.md

@@ -823,16 +823,62 @@ has to be its own commit with its own re-recorded scorecard.
     the anchor rewritten — resuming at it returned updated documents twice,
     caught by draining a collection being updated underneath.
 
-  Still open in M1: the doc-level free list and sessions plumbing (`lsid`
-  accepted). The eight reclamation bugs above were cleared first, as
-  preconditions for the free list rather than as work of their own;
-  command-monitoring (`expectEvents`) landed next, so that the free list and
-  sessions are measured by an instrument that is no longer known to overstate.
+  Still open in M1: the doc-level free list. The eight reclamation bugs above
+  were cleared first, as preconditions for the free list rather than as work of
+  their own; command-monitoring (`expectEvents`) landed next, so that what
+  followed is measured by an instrument no longer known to overstate.
   **A prerequisite the free list must honour**, recorded here while it is
   still being designed: *an offset that was ever a record start must remain a
   record start.* `doc_bytes` reads a `u32` length prefix in place, so an
   offset landing mid-record after a re-split is a garbage-length read rather
   than a wrong answer — and an offsets cursor holds exactly such offsets.
+- **M1 sessions** — *settled and implemented.* `lsid` is parsed, validated and
+  deliberately acted on in no way; `txnNumber`, `startTransaction` and
+  `autocommit` are refused; `endSessions` validates the array it discards.
+  Every code and message was **measured against mongod 8.3.7** with a raw
+  OP_MSG probe, because the driver overwrites `lsid` with its own session and a
+  malformed one cannot be sent through it. Three measurements contradicted the
+  design they were checking:
+  - **Unknown fields inside `lsid` are rejected** (IDLUnknownField 40415). The
+    design said to tolerate them, reasoning that the server tolerates unknown
+    fields everywhere. It does not, here.
+  - **`uid` is accepted** — the hash of the credentials owning the session,
+    which a driver sends as soon as authentication is on. Rejecting it would
+    have broken every command in M7.
+  - **An unknown command with a malformed `lsid` answers CommandNotFound**, so
+    command lookup precedes session validation, which is where the check sits.
+
+  The refusals, so M4 does not reopen them:
+  - **No session registry.** A session here would own nothing: no transactions
+    to scope, no retryable writes (a driver disables them for a standalone),
+    and cursors that outlive their connection for reasons of their own. It
+    would be a mutex on the dispatch path guarding state nothing reads. M4's
+    transaction state machine gets to say what shape it needs.
+  - **`lsid` is not echoed.** Measured: mongod answers a well-formed one with
+    exactly `{ok: 1}`. A driver reads only `$clusterTime` and `operationTime`
+    back, and a standalone sends neither — correctly, since without
+    `operationTime` there is nothing for `afterClusterTime` to attach to and
+    causal consistency stays off.
+  - **`startSession` and `refreshSessions` are not implemented.** Both are real
+    mongod commands, but a driver calls neither — it generates session ids
+    locally — so CommandNotFound is the honest answer. Candidates for M4.
+
+  Five divergences from mongod remain, all deliberate. Three share one cause:
+  mongod keeps a per-command table of which commands accept `txnNumber` at all
+  and answers Location50889 or OperationNotSupportedInTransaction 263 for those
+  that do not, *before* reaching the standalone refusal. We have no such table
+  and give the standalone answer uniformly, so replies are identical for every
+  CRUD command — everything a driver would send these fields on — and differ
+  only on things like `ping`, where mongod is more specific rather than
+  differently right. The other two are `startSession`/`refreshSessions` above,
+  with `commitTransaction` alongside them, reachable only by a client whose
+  write this server has already refused.
+
+  **Effect on the scorecard: exactly zero, and that was the prediction.**
+  194/97/196 before and after. The corpus has no `session` entity, no operation
+  taking a `session` argument, and no `lsid` assertion. The value here is
+  protocol hygiene, not a number — and after Stage 1 a wrongly-refused command
+  would have shown up as a changed event stream rather than silently.
 - **M2 aggregation**: stage/expression tiers, which spec-test files are
   the gate, whether $lookup/$unwind/facet make the first cut.
 - **M4 transactions**: snapshot isolation over mmap (COW vs undo), read