[POC] Feat/onyx readonly experiment

config/eslint/eslint.seatbelt.tsv

@@ -674,6 +674,7 @@
 "../../src/libs/Clipboard/index.ts"	"@typescript-eslint/no-deprecated/document.execCommand"	1
 "../../src/libs/Clipboard/index.ts"	"@typescript-eslint/no-unsafe-type-assertion"	1
 "../../src/libs/Clipboard/index.ts"	"no-restricted-syntax"	1
+"../../src/libs/cloneMutable.ts"	"@typescript-eslint/no-unsafe-type-assertion"	1
 "../../src/libs/ComposerFocusManager.ts"	"@typescript-eslint/no-deprecated/TextInput.State.currentlyFocusedField"	1
 "../../src/libs/ComposerFocusManager.ts"	"@typescript-eslint/no-unsafe-type-assertion"	1
 "../../src/libs/ComposerUtils/index.ts"	"no-restricted-syntax"	1

patches/react-native-onyx/details.md

@@ -0,0 +1,24 @@
+# `react-native-onyx` patches
+
+### [react-native-onyx+3.0.110+001+readonly-types.patch](react-native-onyx+3.0.110+001+readonly-types.patch)
+
+- Reason:
+
+    ```
+    Proof of concept, not for merge. Makes the Onyx cache immutable at the type level, in the two places that
+    matter:
+
+    1. `Onyx.get()` returns `Promise<ReadonlyDeep<OnyxValue<TKey>>>`, so the object it hands back (which IS the
+       cached object, not a copy) cannot be mutated without the compiler objecting. This turns "the result of
+       Onyx.get() MUST NOT be mutated" from a convention reviewers have to remember into a rule tsc enforces.
+    2. Every write input (`OnyxInput`, which funnels into set/merge/multiSet/mergeCollection/setCollection/update)
+       is typed `ReadonlyDeep`, so a value read out of the cache can be passed straight back to a writer without a
+       copy. Without this half, every write path that echoes a read value needs a hand-rolled deep copy.
+
+    Both are widenings on the input side: nothing that passes a value in can break, and Onyx does not mutate write
+    inputs at runtime. See https://github.com/Expensify/App/issues/71206 for the measurements.
+    ```
+
+- Upstream PR/issue: https://github.com/Expensify/App/issues/71206
+- E/App issue: https://github.com/Expensify/App/issues/71206
+- PR introducing patch: experiment branch, no PR

patches/react-native-onyx/react-native-onyx+3.0.110+001+readonly-types.patch

@@ -0,0 +1,38 @@
+diff --git a/node_modules/react-native-onyx/dist/Onyx.d.ts b/node_modules/react-native-onyx/dist/Onyx.d.ts
+index b9848a6..c4045b1 100644
+--- a/node_modules/react-native-onyx/dist/Onyx.d.ts
++++ b/node_modules/react-native-onyx/dist/Onyx.d.ts
+@@ -1,4 +1,5 @@
+ import * as Logger from './Logger';
++import type { ReadonlyDeep } from 'type-fest';
+ import type { CollectionKeyBase, ConnectOptions, InitOptions, OnyxKey, OnyxMergeCollectionInput, OnyxMergeInput, OnyxMultiSetInput, OnyxSetCollectionInput, OnyxSetInput, OnyxUpdate, OnyxValue, SetOptions } from './types';
+ import type { Connection } from './OnyxConnectionManager';
+ /** Initialize the store with actions and listening for storage events */
+@@ -170,7 +171,7 @@ declare function setCollection<TKey extends CollectionKeyBase>(collectionKey: TK
+  * @param key ONYXKEY to read, either a collection key or a single key
+  * @returns The current value, or `undefined` if the key has none.
+  */
+-declare function get<TKey extends OnyxKey>(key: TKey): Promise<OnyxValue<TKey>>;
++declare function get<TKey extends OnyxKey>(key: TKey): Promise<ReadonlyDeep<OnyxValue<TKey>>>;
+ declare const Onyx: {
+     METHOD: {
+         readonly SET: "set";
+diff --git a/node_modules/react-native-onyx/dist/types.d.ts b/node_modules/react-native-onyx/dist/types.d.ts
+index cb91605..f974c46 100644
+--- a/node_modules/react-native-onyx/dist/types.d.ts
++++ b/node_modules/react-native-onyx/dist/types.d.ts
+@@ -1,4 +1,4 @@
+-import type { Merge } from 'type-fest';
++import type { Merge, ReadonlyDeep } from 'type-fest';
+ import type OnyxUtils from './OnyxUtils';
+ import type { OnyxMethod } from './OnyxUtils';
+ import type { FastMergeReplaceNullPatch } from './utils';
+@@ -219,7 +219,7 @@ type OnyxCollectionInputValue<TOnyxValue> = OnyxInputValue<Record<string, TOnyxV
+  * Setting a key to `null` will remove the key from the store.
+  * `undefined` is not allowed for setting values, because it will have no effect on the data.
+  */
+-type OnyxInput<TKey extends OnyxKey> = OnyxInputValue<NullishDeep<KeyValueMapping[TKey]>>;
++type OnyxInput<TKey extends OnyxKey> = ReadonlyDeep<OnyxInputValue<NullishDeep<KeyValueMapping[TKey]>>>;
+ /**
+  * Represents a mapping object where each `OnyxKey` maps to either a value of its corresponding type in `KeyValueMapping` or `null`.
+  *