Support export-prefixed env vars and parsing

[MikeGarde]

• Add EnvValue.exported flag (default false) and propagate to scalar/list values; initialize in all constructors.
• Implement robust multi-line list parsing with bracket-depth, export prefix support, key validation, and error handling; introduce helpers and propagate exported flag through values.
• Centralize and preserve export-prefix handling in set_value to keep rewritten files sourceable.
• Replace legacy end-line detection with the new parsing approach.
• Extend release notes with a Docker installation snippet and arch-aware dotenv-cli download logic.
• Add regression tests for complex lists, trailing comments, export semantics, and export-preservation behavior across set/delete flows.

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/env_parser.rs	95.12%	4 Missing ⚠️
src/env_object.rs	50.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/handlers/set_value.rs	100.00% <100.00%> (ø)
src/env_object.rs	83.33% <50.00%> (-0.76%)	⬇️
src/env_parser.rs	93.61% <95.12%> (+1.22%)	⬆️

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