fix(ci): resolve latent PHPCS + JS lint failures

Latent debt surfaced by the reusable PHPCS job (previously continue-on-error).

PHPCS fixes (all pre-existing, not introduced by this PR):
- Add @package PeptideReconstitutionCalculator to file docblocks in all 12 PHP
  files (previously missing; phpcbf cannot insert missing docblock tags).
- Replace two short-ternary ?: operators in class-prc-preset-provider.php with
  full ternary (Universal.Operators.DisallowShortTernary.Found; lines 136+152).

JS lint fix:
- Add package.json + package-lock.json (no deps; lint script = node --check)
  so the reusable lint-js job's npm ci / cache: npm steps succeed. The
  calculator ships vanilla JS with no build toolchain — package.json is the
  minimal stub the reusable workflow requires for has_js: true.

Agent-Session: claude/adopt-ci-20260616

includes/class-prc-deactivator.php

@@ -5,6 +5,7 @@
  * What: Runs on plugin deactivation — flushes rewrite rules.
  * Who calls it: register_deactivation_hook in bootstrap.
  * Dependencies: None.
+ * @package PeptideReconstitutionCalculator
  */
 
 declare(strict_types=1);