Remove unused declarations and const-related lint debt

[fpigeonjr]

Parent

#580

What to build

Remove unused declarations and convert never-reassigned bindings to constants across independently testable component areas. Preserve public exports and the raw-source package contract while eliminating the current no-unused-vars and prefer-const debt.

Acceptance criteria

• Unused imports, variables, parameters, and declarations are removed or intentionally named/configured
• Never-reassigned bindings use const where behavior is unchanged
• Public exports and frozen deep-import paths remain intact
• Relevant component tests and the full test-app suite remain green
• no-unused-vars and prefer-const are promoted to errors when their debt reaches zero
• The warning baseline is lowered to reflect the cleanup

Blocked by

• Apply safe ESLint autofixes and lower the baseline #582