Reduce reliance on the IMMUTABLE tnum bit in the plain list code

[fingolfin]

Preparation for #6025, which moves the mutability of lists and records from the tnum into an object flag. These commits stop a few places from handling the IMMUTABLE bit by hand. No change in behaviour.

• ShallowCopyPlist and ElmsPlistDense use MUTABLE_TNUM instead of subtracting IMMUTABLE or comparing against ... + IMMUTABLE.
• KTNumPlist and KTNumHomPlist return the mutable tnum. TypePlistHomHelper, which needs the bit to pick the type, adds it back from IS_MUTABLE_OBJ. This continues kernel: avoid some uses of IMMUTABLE in plist.c #6024 and is the one commit that needs a careful look.
• IS_PLIST_MUTABLE is replaced by IS_MUTABLE_OBJ and removed. I found no use of it in the distributed packages.
• NEW_PLIST_IMM and NEW_PLIST_WITH_MUTABILITY go through MakeImmutableNoRecurse instead of or-ing the bit into the tnum.

AI disclosure: Claude Code (Opus 5.5)

[codecov]

Codecov Report

❌ Patch coverage is 90.24390% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.54%. Comparing base (482ebce) to head (16a3fc4).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/plist.c	88.88%	3 Missing ⚠️
src/listfunc.c	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6707      +/-   ##
==========================================
- Coverage   79.56%   79.54%   -0.03%     
==========================================
  Files         684      684              
  Lines      295665   295668       +3     
  Branches     8656     8656              
==========================================
- Hits       235259   235190      -69     
- Misses      58582    58654      +72     
  Partials     1824     1824              

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