Refactor internal state: scope state env + callable S7 R2F handlers

[t-kalinowski]

This refactors internal object/state management to reduce ad-hoc attributes and make compiler state easier to reason about.

Changes

• Keep quickr_scope as an environment (reference semantics), but move mutable per-scope state into a dedicated attr(scope, "state") env with explicit helpers.
• Introduce callable S7 R2FHandler (inherits from class_function) to store handler metadata as properties (replacing attr(handler, ...)).
• Move Fortran lowering metadata off arbitrary attributes onto explicit S7 properties.

Testing

• R -q -e "Sys.setenv(TESTTHAT_PARALLEL="false"); rcmdcheck::rcmdcheck(error_on="warning")" (Status: OK; 0 errors, 0 warnings, 0 notes)
• Coverage maintained (covr was >= baseline before/after refactor).

Notes

• Handler call sites now read handler metadata via S7 properties (with compatibility for non-registered functions where needed).

[codecov]

Codecov Report

❌ Patch coverage is 97.25275% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.93%. Comparing base (2f8206c) to head (51e4988).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
R/r2f-aab-core.R	90.90%	2 Missing ⚠️
R/error-handling.R	80.00%	1 Missing ⚠️
R/r2f-aaa-registry.R	92.30%	1 Missing ⚠️
R/scope.R	96.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
+ Coverage   92.89%   92.93%   +0.03%     
==========================================
  Files          27       28       +1     
  Lines        5859     5941      +82     
==========================================
+ Hits         5443     5521      +78     
- Misses        416      420       +4     

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