fix: document CPU-only constraint on determineLocalComposing

Every commit/delete/selection path calls determineLocalComposing inside
runBlocking on the IME main thread. Safe today because all providers
(Latin BreakIterator, Han shape engine) are CPU-only, but a future
provider doing I/O would ANR on every keystroke.

Pinned the invariant at both the interface contract (NlpProviders.kt)
and the call site (AbstractEditorInstance.kt).

Author: SysAdminDoc

app/src/main/kotlin/dev/patrickgold/florisboard/ime/editor/AbstractEditorInstance.kt

@@ -351,6 +351,8 @@ abstract class AbstractEditorInstance(context: Context) {
         return editorInfo.isRichInputEditor && !editorInfo.inputAttributes.flagTextNoSuggestions
     }
 
+    // INVARIANT: must remain CPU-only (no I/O, no suspend delay). Called inside
+    // runBlocking on the IME main thread from every commit/delete/selection path.
     private suspend fun determineLocalComposing(
         textBeforeSelection: CharSequence, localLastCommitPosition: Int
     ): EditorRange {

app/src/main/kotlin/dev/patrickgold/florisboard/ime/nlp/NlpProviders.kt

@@ -203,6 +203,10 @@ interface SuggestionProvider : NlpProvider {
     /**
      * When initializing composing text given a new context, the suggestion engine determines the composing range.
      * The default behavior gets the last word according to the current subtype's primaryLocale.
+     *
+     * **CONSTRAINT:** implementations MUST be CPU-only — no disk I/O, no network, no suspend
+     * delay. This is called inside `runBlocking` on the IME main thread on every keystroke.
+     *
      * @param subtype The current subtype used to determine word or character boundary.
      * @param textBeforeSelection The text whose end we want to compose.
      * @param breakIterators cache of BreakIterator(s) to determine boundary.