SEA: preserve raw strings with decrypt skipParse option

[tmsteph]

What changed

Adds an opt-in skipParse option to SEA.decrypt so callers can recover plaintext strings exactly as encrypted, even when the string happens to look like JSON.

The default behavior is unchanged for backwards compatibility: decrypted plaintext still passes through SEA.opt.parse unless skipParse is explicitly enabled.

Example:

const enc = await SEA.encrypt('[hello there]', pair);
const dec = await SEA.decrypt(enc, pair, null, { skipParse: true });
// dec === '[hello there]'

This updates both the canonical sea.js bundle and extracted sea/decrypt.js, exposes the callback/options arguments in the TypeScript declarations, and adds regression coverage for JSON-looking string payloads.

Fixes #1418.

Validation

• npm run testsea: 36 passing, 1 pending, including the new skipParse regression.
• test/sea/nodeauth.js: all 5 visible scenarios passed on the first clean run. The harness intentionally calls process.exit(-1) in its after hook, so it exits with status 255 after the tests.
• git diff --check: clean.