Master Filesystem Valhalla compatibility

[mbien]

first commit fixes the internal reference tracking WeakHashMap:

• WeakHashMap keys, which are used to track the lifecycle of BaseFileObj, must be identity objects.
• this can be also seen as a bugfix, since Integers are bad reference strength indicators due to the internal Integer boxing cache - some are always strongly reachable

second commit is for module cleanup

• language level bump, overrides and simplifications.

tested using:

26-jep401ea2+1-1 (don't forget -J--enable-preview)

NB starts and projects load at least, but more work may be needed in other areas (did only very basic testing).

value object JEP: https://openjdk.org/jeps/401

meta issue #8256

[mbien]

The errors from these issues must have been silently thrown because Netbeans 27 started just fine for me. The IDE did crash everytime the editor's context menu was opened though.

Of course Netbeans was also highlighted perfectly fine code as having errors. I could've sworn removing nb-javac worked around that last EA build but maybe not.

This is not about editor support for JDK 26 or any editor features regarding value types. This PR so far is just here to allow (very) early testing of running NB on ea JDKs which have the JEP 401 implementation enabled.

[mbien]

while renovating the code I found more issues in FileObjectFactory but addressing them would be out of scope of this PR.

E.g:

• the list entry of the internal cache can't shrink atm.
• there is also an instance identity problem which could cause the WeakHashMap to remove a full List entry even though some of the list items are still alive
• FileObjectFactory::toString will throw CCE if the cache contained a list (printing all entries might also not be reasonable)

I might do some more cleanup as preparation for the fixes though.

[lahodaj]

Looks sensible to me.

[mbien]

will rebase on top of #8799 so that we can test both valhalla related updates using the same dev build

planning to integrate this this week, dependent on feedback. Thanks for the review!

[mbien]

all green, ran a indexer smoke test which indexed all ~850 nb modules using 26-jep401ea and checked a few things manually (eg some dialogs due to #8799) everything worked as before.

thanks for the review -> merging