feat(theme): dark variant

[hkimw]

Refs #125.

Summary:

• Adds pccx SystemVerilog Dark beside the existing light theme.
• Uses the aperture-mark blue and ink colors with a darker editor/workbench base.
• Adds manifest wiring, README listing, and type-only JSON theme validation.

Validation:

• node editors/vscode-prototype/test/extension-manifest.test.mjs
• jq empty editors/vscode-prototype/package.json editors/vscode-prototype/themes/pccx-systemverilog-light-color-theme.json editors/vscode-prototype/themes/pccx-systemverilog-dark-color-theme.json
• git diff --check

Scope guard:

• Theme variant only.
• No command, activation, runtime, LSP, provider, API, ABI, or publishing metadata changes.