test: add StoreStress harness and contention regression tests

Author: steipete

Scripts/StoreStress/.gitignore

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+Package.resolved

Scripts/StoreStress/Package.swift

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+// swift-tools-version: 6.2
+import PackageDescription
+
+let package = Package(
+    name: "StoreStress",
+    platforms: [
+        .macOS(.v14),
+    ],
+    dependencies: [
+        .package(name: "CodexBar", path: "../.."),
+    ],
+    targets: [
+        .executableTarget(
+            name: "StoreStress",
+            dependencies: [
+                .product(name: "CodexBarCore", package: "CodexBar"),
+            ],
+            path: ".",
+            exclude: ["Package.swift", "README.md"],
+            swiftSettings: [
+                .swiftLanguageMode(.v5),
+            ]),
+    ])

Scripts/StoreStress/README.md

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+# StoreStress
+
+Adversarial crash, contention, corpus-rebuild, WAL, and descriptor harness for the SQLite-backed
+`CostUsageStore`. It is a separate package so its internal test access never enters normal CodexBar builds.
+
+Build the optimized harness from this directory:
+
+```sh
+swift build -c release -Xswiftc -enable-testing
+```
+
+Run `swift run -c release -Xswiftc -enable-testing StoreStress --` without arguments to print the available
+subcommands. Every store/cache argument should point at a disposable temporary directory. `rebuild` and
+`incremental` accept an optional sessions root, which makes it possible to measure a read-only corpus snapshot
+without writing to the real Codex session or CodexBar cache directories.