Extract constructive.core package to sub/ directory

[Copilot]

Implements first step of package modularization by extracting core infrastructure into standalone constructive.core package in sub/ directory.

Package Structure

Created sub/constructive.core/ containing:

• Core infrastructure (34 files): construct(), construct_multi(), construct_diff(), helper utilities, options system, attribute repair, deparse functions
• Base type methods (18 files): Constructors for NULL, atomic vectors, list, array, matrix, environment, function, language objects, pairlist, dots, externalptr, S4/object types
• C++ code: External pointer and environment handling (updated for constructive.core namespace)
• Tests: 26 test files with snapshots covering core functionality
• Metadata: DESCRIPTION, LICENSE, README, .Rbuildignore

Implementation Notes

• Package is self-contained and buildable independently
• Main constructive package unchanged (code duplication maintained for now)
• Updated .Rbuildignore to exclude sub/ from main package builds
• Dependencies: cli, diffobj, methods, rlang, waldo

Next Steps

Future PRs will:

1. Make main package depend on constructive.core
2. Remove duplicated code from main package
3. Extract additional packages (constructive.base, constructive.ggplot2, etc.)

Addresses #638, supports #581 roadmap.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Extract constructive.core package</issue_title> <issue_description>See parent issue. The package should live in the sub directory in the root of this repository. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes cynkra/constructive#638

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