test(engine): property-based query-correctness invariants over generated graphs

Adds a proptest harness (new dev-dep) that generates small graphs whose Person
and Company keys are drawn from a shared 5-key alphabet, so cross-type id
collisions, cycles, and self-loops arise by search rather than from one
hand-built fixture. Three invariants:

- prop_expand_indexed_eq_csr: csr == indexed == auto over knows{1,3} (same-type,
  cycles) and worksAt{1,2} (cross-type, collision-prone) from every start.
- prop_results_subset_of_existing_nodes: no phantom rows (catches over-emission
  even if both modes are wrong identically).
- prop_antijoin_partitions_persons: not{worksAt} and its complement are disjoint
  and cover all persons.

Verified the guard bites: neutering the cross-type hop cap makes
prop_expand_indexed_eq_csr fail and proptest shrinks it to persons["c","e"] /
companies["b","c"] — the cross-type collision class the hand-built fixture
only sampled once. Tests are sync + #[serial] (per-case runtime; the mode test
writes OMNIGRAPH_TRAVERSAL_MODE).

Cargo.lock

@@ -4611,6 +4611,7 @@ dependencies = [
  "object_store 0.12.5",
  "omnigraph-compiler",
  "omnigraph-policy",
+ "proptest",
  "regex",
  "reqwest",
  "serde",
@@ -5125,6 +5126,25 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "proptest"
+version = "1.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4b45fcc2344c680f5025fe57779faef368840d0bd1f42f216291f0dc4ace4744"
+dependencies = [
+ "bit-set",
+ "bit-vec",
+ "bitflags",
+ "num-traits",
+ "rand 0.9.2",
+ "rand_chacha 0.9.0",
+ "rand_xorshift",
+ "regex-syntax",
+ "rusty-fork",
+ "tempfile",
+ "unarray",
+]
+
 [[package]]
 name = "prost"
 version = "0.14.3"
@@ -5186,6 +5206,12 @@ dependencies = [
  "cc",
 ]
 
+[[package]]
+name = "quick-error"
+version = "1.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a1d01941d82fa2ab50be1e79e6714289dd7cde78eba4c074bc5a4374f650dfe0"
+
 [[package]]
 name = "quick-xml"
 version = "0.37.5"
@@ -5357,6 +5383,15 @@ dependencies = [
  "rand 0.9.2",
 ]
 
+[[package]]
+name = "rand_xorshift"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "513962919efc330f829edb2535844d1b912b0fbe2ca165d613e4e8788bb05a5a"
+dependencies = [
+ "rand_core 0.9.5",
+]
+
 [[package]]
 name = "rand_xoshiro"
 version = "0.7.0"
@@ -5756,6 +5791,18 @@ version = "1.0.22"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b39cdef0fa800fc44525c84ccb54a029961a8215f9619753635a9c0d2538d46d"
 
+[[package]]
+name = "rusty-fork"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cc6bf79ff24e648f6da1f8d1f011e9cac26491b619e6b9280f2b47f1774e6ee2"
+dependencies = [
+ "fnv",
+ "quick-error",
+ "tempfile",
+ "wait-timeout",
+]
+
 [[package]]
 name = "ryu"
 version = "1.0.23"
@@ -6743,6 +6790,12 @@ dependencies = [
  "web-time",
 ]
 
+[[package]]
+name = "unarray"
+version = "0.1.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "eaea85b334db583fe3274d12b4cd1880032beab409c0d774be044d4480ab9a94"
+
 [[package]]
 name = "unicase"
 version = "2.9.0"