fix: clear watcher registry in disableAll

[zloglevel]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	N/A
License	MIT
Doc PR	N/A

Summary

God.watch._watchers is initialized as an object keyed by pm_id, but
God.watch.disableAll() previously attempted to remove entries with
Array.prototype.splice().

When the watcher registry is non-empty, the first watcher is closed and the method then throws:

TypeError: watchers.splice is not a function

This prevents the remaining watchers from being closed and removed.

Changes

• Replace the invalid splice() call with Object.keys() iteration and
  property deletion.
• Add a regression test with two fake watchers.
• Verify that each watcher is closed exactly once.
• Verify that the watcher registry is empty after disableAll() completes.

Testing

npx mocha --exit --bail test/interface/watcher.spec.mocha.js

Result: 1 passing.

[CLAassistant]

All committers have signed the CLA.

[zloglevel]

@Unitech Could you please take a look when you have a chance? Thanks!