Derived Contentlet map properties can silently overwrite declared fields (shortyId, titleImage, hasTitleImage, urlMap)

Problem Statement

The Contentlet map is one flat namespace holding both stored field values and derived
properties computed at read time. FieldFactoryImpl.RESERVED_FIELD_VARS stops a Content Type
declaring a field whose variable collides with a derived key — but it does not cover all of them.

Measured against the current list (34 reserved keys):

For any unreserved key, a customer can legally declare a field with that variable and every
transform will silently overwrite the stored value with the derived one. That is exactly the
defect reported in #37584, where hostName collided with the Host Content Type's own "Site Key"
field and every Site reported System Host.

Severity / Impact: silent data misreporting on the REST read path for any Content Type that
declares one of these variables. Unlike #37584 there is no shipped Content Type that does so today,
so this is latent rather than active — which is why it was kept out of #37589's scope.

Suggested Fix

Two options, not mutually exclusive:

1. Extend the guard. DefaultTransformStrategy.declaresField(type, key) (added in #37589)
   already expresses the rule "a declared field beats a derived property". Apply it to the
   remaining keys, with the same sentinel fallback so the map keeps its non-null guarantee.
2. Close the hole at the source. Add the unreserved derived keys to RESERVED_FIELD_VARS so
   new Content Types cannot declare them. This does not help Content Types that already have
   such a field — and would make them unsaveable — so it complements (1) rather than replacing it.

Worth recording the precedence rule as an ADR in dotCMS/platform-adrs: stored field values take
precedence over derived properties in the Contentlet map. Proposed, not written, in #37589's plan.

Notes

Follow-up to #37584 / #37589, which fixed the one active instance (hostName) and deliberately
left the general rule for a separate change. The legacy ContentletToMapTransformer carries the
same writes and needs the same treatment.

Start with DefaultTransformStrategy.addCommonProperties and declaresField, then compare the corresponding writes in the legacy ContentletToMapTransformer. Review FieldFactoryImpl.RESERVED_FIELD_VARS and the follow-up issues for the intended precedence rule; done means declared stored values are not overwritten for shortyId, titleImage, hasTitleImage, or urlMap, while the map retains its non-null guarantee.