Make VarNamedVector the default metadata type

[mhauru]

What it says on the tin.

Metadata stuff is renamed as UntypedLegacyVarInfo and (un)typed_legacy_varinfo. UntypedVarInfo and (un)typed_varinfo become synonyms for the UntypedVectorVarInfo and (un)typed_vector_varinfo.

The diff is currently a mess because this builds on #1102. Opening this early to run all of CI on GHAs.

This doesn't need to be merged into breaking, since changes are internal. Considering how these code paths are invoked by everything we do in Turing.jl though, I would be surprised if something didn't break, and thus I think a breaking version bump is the right choice.

Whether these changes should even be released before VarNamedTuple changes are in can be discussed.

[github-actions]

Benchmark Report for Commit a76e366

Computer Information

Julia Version 1.11.7
Commit f2b3dbda30a (2025-09-08 12:10 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 4 × AMD EPYC 7763 64-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver3)
Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)

Benchmark Results

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.89%. Comparing base (6dc7c02) to head (a76e366).

Files with missing lines	Patch %	Lines
src/varinfo.jl	33.33%	18 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (6dc7c02) and HEAD (a76e366). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (6dc7c02)	HEAD (a76e366)
	11	6

Additional details and impacted files

@@              Coverage Diff              @@
##           breaking    #1105       +/-   ##
=============================================
- Coverage     81.23%   32.89%   -48.35%     
=============================================
  Files            40       40               
  Lines          3805     3791       -14     
=============================================
- Hits           3091     1247     -1844     
- Misses          714     2544     +1830     

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