feat(run-store,run-engine): freeze the completed-waitpoints record and resolver contract (#4760)

Builds on
[#4754](#4754), which
added the store this contract belongs to.

## Why

Two migrations are moving to Redis in parallel, and execution snapshots
reference completed waitpoints across the boundary between them. If the
record shape is agreed only once both halves are built, the correction
lands mid-rollout: dual-write is live, real keys are in Redis, and
changing the entry format then means two versions of the entry
coexisting plus a migration for whatever was already written. Agreeing
it now, while nothing writes a pointer, makes that same correction a
type edit.

The reserved-and-empty field is the same argument one level down. The
entry format is what dual-write writes, so adding a field to it later
splits the format in two. Reserving it before any write means the format
never changes after writes begin.

## Summary

Adds the type contract for carrying completed waitpoints alongside the
Redis-backed execution-snapshot store: a `{cycleSeq, count}` pointer on
the snapshot entry, the record shape that pointer resolves to, and the
read-time resolver signature. Nothing constructs or reads a pointer yet,
so this is inert on merge.

The record shape has to reproduce
`enhanceExecutionSnapshotWithWaitpoints` field for field, because that
is what the executor consumes. A conformance test runs the real function
against a reference resolver over an exhaustive grid of 6144 input
combinations, derived from every `Waitpoint` column the function reads
rather than hand-picked.

## Design

`completedWaitpoints` is reserved on the entry type and always unset.
`append()` rejects a set value, because the pointer's physical home is
the `<snapshotId>#c` sidecar field rather than the entry JSON. The
append script mints both halves after the client serializes the entry,
and the entry JSON has to stay byte-identical to the Postgres row so the
two can be compared during a dual-write rollout.

Two rules are worth calling out, both found by making the test fail
rather than by reading the code:

* `records` is the authoritative waitpoint set, not `order`. Only batch
waits carry an index, so `order` is empty for a single `triggerAndWait`
while the Postgres join still holds the id. Comparing id sets over
`order` would serve the previous wait cycle's records.
* `deriveFromRun` requires a non-null `completedByTaskRunId`.
`Waitpoint.completedByTaskRun` is `onDelete: SetNull`, so an orphaned
RUN waitpoint keeps its output with no run left to derive from. Those
records carry their output inline instead.

`tsconfig.freeze-test.json` typechecks the conformance test, which the
package build config excludes. Without it, renaming a field in the
frozen type compiles clean and every test stays green, so the literal
assertions in the test would only pin the test's own writer.

## Fixes carried along

Auditing the contract surfaced three defects in the append script, each
with a regression test that fails when the fix is reverted:

* A new wait cycle now clears any `records` left on a reused key. A
`seq` counter lost to eviction can re-mint a `cycleSeq` whose key still
holds another cycle's records, and `order` and `count` are overwritten
together, so the mismatch check could not see the drift.
* A carry-forward now attaches a pointer only if the current keyspace
incarnation actually minted that cycle. The previous key-exists check
adopted a dead incarnation's records under a count that agreed with
them, reporting no mismatch.
* The cycle-key size metric now counts `records`, not only `order`. It
reported 7 bytes for a 20 KB key, so the high-water log could never fire
on the field that grows.

Author: d-cs

internal-packages/run-engine/package.json

@@ -43,7 +43,7 @@
   },
   "scripts": {
     "clean": "rimraf dist",
-    "typecheck": "tsc --noEmit -p tsconfig.build.json",
+    "typecheck": "tsc --noEmit -p tsconfig.build.json && tsc --noEmit -p tsconfig.freeze-test.json",
     "test": "vitest --sequence.concurrent=false --no-file-parallelism",
     "test:coverage": "vitest --sequence.concurrent=false --no-file-parallelism --coverage.enabled",
     "build": "pnpm run clean && tsc -p tsconfig.build.json",