refactor(oxlint): enable no-undef

[secustor]

Changes

Enables the no-undef rule at error. The rule was previously unusable because .oxlintrc.json declared no env/globals, so every reference to a runtime global was reported (36,672 diagnostics).

Adds the missing environment configuration:

• Top level env: node (Node.js + CommonJS globals such as process, Buffer, console, require, module, __dirname, needed by lib/, tools/ and the .github scripts) and es2025, matching "lib": ["ES2025"] in tsconfig.json.
• Test-only globals are scoped to the existing ["**/*.spec.ts", "test/**"] override instead of polluting the top level: the built-in vitest env covers describe, it, test, expect, expectTypeOf, vi, beforeEach/afterEach/ beforeAll/afterAll, mirroring "types": ["vitest/globals"].
• The repo's own fixtures global, installed in test/setup.ts via Object.defineProperty(global, 'fixtures', ...), is declared as a readonly global in the same override.

Context

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• This closes an existing Issue, Closes: #
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