fix: lint cleanup — JSON import compat, unused vars, HLS_CONFIG freeze

- Replace 'import ... with { type: "json" }' with fs-based JSON.parse
  (ESLint espree cannot parse JSON import attributes)
- Prefix unused catch-clause variables with _ (e -> _e)
- Prefix unused hls.js event data params with _ (data -> _data)
- Freeze HLS_CONFIG with Object.freeze for defensive immutability

Lint: 0 errors (3 cosmetic _e warnings in catch blocks)
Tests: 44/44 passed
Build: 9 modules, 201ms

__tests__/data_flow.test.mjs

@@ -1,7 +1,10 @@
 import { describe, it } from './runner.mjs';
 import Ajv from 'ajv';
 import addFormats from 'ajv-formats';
-import schema from '../src/viewer/schema.json' with { type: 'json' };
+import { readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import { dirname, join } from 'node:path';
+const schema = JSON.parse(readFileSync(join(dirname(fileURLToPath(import.meta.url)), '../src/viewer/schema.json'), 'utf8'));
 
 const ajv = new Ajv({ strict: false });
 addFormats(ajv);