Migration guide: untouched fields, throws after dispose, reset, field streams, disposing subforms

[KamilSztandur]

Problem

Two ports from 0.1.x compiled fine and then surprised people at runtime, in the same five places. The guide either did not mention them or hid them in one sentence in the middle of a section:

• a field loaded from the server stopped reacting to the fields it depends on, silently;
• setValue after an await threw on a screen that had closed;
• resetAll() left fields read-only and left the validation mode on;
• porting field.stream.distinct().listen(...) to addListener brought back the old bug of reacting to markReadOnly();
• a dynamic list of subforms piled up detached controllers until the parent died.

Solution

Each one gets a checklist item at the top of MIGRATION.md and a section with a before/after: the untouched-field gate with the validate()-from-a-listener recipe and its limits, the full list of what throws after dispose() with the isDisposed guard, addRelation as the first port for a field stream, and removeSubform plus dispose() for a section that is gone for good. The short copy on the website gets the same bullets.

What it is like now

Someone porting an edit screen reads the checklist, finds "review subscribeToFields on prefilled fields" and "guard late writes", and does not spend an afternoon in the debugger.

Design decisions

None in the package. The behaviours described are the ones 0.2.x already has, and the guide now says so up front. The listener recipe relies on the validate() fix in #96, so this merges after it.

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