fix: lazy initialization for clipboard enable flag

[cxxxr]

Summary

• Fix clipboard initialization issue on Nix builds
• Change *enable-clipboard-p* from defparameter to defvar for lazy initialization
• Move platform detection logic into enable-clipboard-p function

Problem

On Nix builds, the clipboard enable flag was incorrectly initialized at compile time rather than runtime. This caused the platform detection (macOS SBCL version check, WSL detection) to evaluate in the build environment instead of the execution environment, resulting in incorrect clipboard behavior.

Solution

Defer the platform detection to runtime by initializing the flag lazily on first access to enable-clipboard-p.

[code-contractor-app]

✅ Code Contractor Validation: PASSED

✓ Code Contractor Validation Result
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📊 Statistics:
  Files Changed:    1
  Lines Added:      11
  Lines Deleted:    5
  Total Changed:    16
  Delete Ratio:     0.31 (31%)

Status: PASSED ✅

🤖 AI Providers:
  - codex — model: (Codex default)

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📋 Validation Rules:

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