feat: standalone visual context CSS generation

[amanabiy]

Issue #, if available:

This change allows new visual contexts to be compiled to separate CSS files that are only included in the bundle when the component is imported, making context addition scalable without growing the base bundle.

Description of changes:

Adds a destination field to the Context interface. When set, the context is excluded from the base styles and compiled to its own CSS file at the specified path. The component that uses the context imports this file directly.

// In the context definition:
{
  id: 'navigation-bar',
  selector: '.awsui-context-navigation-bar',
  destination: 'visual-contexts/navigation-bar.scoped.css',
  defaultMode: 'dark',
  tokens: { ... }
}

// In the component:
import '../visual-contexts/navigation-bar.scoped.css';

Tested with

• components PR #4477 (last commit) consuming directly from this branch
• Generated file verified: correct token overrides, :not(#\9) specificity, excluded from base styles, existing contexts unchanged

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[codecov]

Codecov Report

❌ Patch coverage is 97.24771% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.63%. Comparing base (beaa124) to head (fd212d8).

Files with missing lines	Patch %	Lines
src/build/tasks/standalone-contexts.ts	89.65%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #180      +/-   ##
==========================================
+ Coverage   97.58%   97.63%   +0.05%     
==========================================
  Files          48       49       +1     
  Lines        2651     2751     +100     
  Branches      545      530      -15     
==========================================
+ Hits         2587     2686      +99     
- Misses         64       65       +1     

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