Avoid stack allocation for large locals

[t-kalinowski]

Summary:

• Emit large local arrays as deferred-shape allocatable locals + allocate() at subroutine entry (avoids flang stack overflows)
• Add allocatable-locals tests

Testing:

• R -q -e "devtools::test()"

[codecov]

Codecov Report

❌ Patch coverage is 96.46018% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.89%. Comparing base (1de5107) to head (b2360ee).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
R/r2f-closures.R	92.68%	3 Missing ⚠️
R/manifest.R	98.36%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
+ Coverage   92.78%   92.89%   +0.11%     
==========================================
  Files          27       27              
  Lines        5754     5859     +105     
==========================================
+ Hits         5339     5443     +104     
- Misses        415      416       +1     

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