Run Metalama 2027's ImmutableType and Durable analyzers over the aspects and the fabric #221

Goal

Run Metalama 2027.0's analyzers for aspect authors over TypeKit's aspects, fabric and validators, and fix what they report. Needs #173.

Metalama 2027.0 ships two contracts, checked by analyzers in Metalama.Framework itself, both reporting warnings only (release notes, "Analyzers for aspect authors"):

• [ImmutableType]: an aspect, fabric or validator that assigns a field outside its constructor.
• [Durable]: an object held across compilations that could keep a Compilation, a syntax tree or a symbol alive (LAMA0870 to LAMA0879).

The repository builds with warnings as errors, so these fail the build until they're answered.

Done when

• The Generators project builds on 2027.0 with both analyzers active and no suppression. Any suppression has a measured reason in the code.
• Each finding is fixed in this issue's PR, or filed as a bug of its own if it changes behaviour. The Generators: CodeAnnotations keeps Metalama attribute constructions in static fields #90 CodeAnnotations statics fix is re-checked, since it is the case [Durable] was built to catch (Metalama issue 2307).
• The [Durable] placeholder on _value in ValueObjectAspect.Templates.cs returns, if Move the Generators package to Metalama 2027.0 in 2.4.0 (decisions 120 and 129) #173 hasn't brought it back already.