fix: preserve explicit priority 0 in facility ordering

[AI-Reviewer-QS]

Summary

• Fix facs() treating explicit priority 0 as unset due to JavaScript falsy semantics (!0 === true)
• The if (!p[5]) p[5] = 1 check on base.js:152 mutates facilities registered with priority 0 to priority 1, corrupting the intended startup and shutdown ordering
• Also fix f[5] || 0 in start() and stop() sort comparators which have the same issue

Problem

When a facility is registered with explicit priority 0 via setInitFacs():

this.setInitFacs([
  ['fac', 'hp-svc-facs-store', 's0', 's0', { storeDir }, 0],  // priority 0
  ['fac', 'hp-svc-facs-net', 'r0', 'r0', () => ({ fac_store: this.store_s0 }), 1],
  ['fac', 'svc-facs-logging', 'l0', 'l0', { name, mixin: this.loggerMixin.bind(this) }, 2]
])

The facs() method mutates priority 0 to 1 because !0 is true in JavaScript. After start(), the store facility and net facility share the same priority (1), making shutdown order non-deterministic. The intended shutdown order (log -> net -> store) is not guaranteed.

Fix

Replace falsy checks with nullish checks:

Location	Before	After
facs() default	if (!p[5]) p[5] = 1	if (p[5] == null) p[5] = 1
start() sort	f[5] || 0	f[5] ?? 0
stop() sort	(f[5] || 0) * -1	(f[5] ?? 0) * -1

Test plan

• Added unit tests verifying priority 0 is preserved after facs() runs
• Added unit test verifying correct shutdown order (log -> net -> store)
• Existing tests pass
• Linter passes