feat(engagement): let a module ship its own templates and rules (Phase 11b)

Phase 11a declared 24 triggers and stopped where the plan said it would. Standing
11b up found that the next sentence — "24 rules, all enabled = 0; bespoke template
bodies" — described work with no mechanism to land in: templateSeeds.js and
coreRules.js are core files with core arrays in them, and there was no
registerTemplates or registerRules anywhere in registries.js.

So a module could say what an event's payload was and could never say what the
mail should read like. That is tolerable for one trigger and not for a catalogue,
and it is decisive once the bodies carry domain prose core must not contain (§5.2).

Adds api.registerEngagementSeeds({ templates, ruleGroups }) — MODULE_API 1.9.0.
The module supplies data; core keeps seedOne's customized skip, its seed_version
comparison and the block registry's validation, which is the whole argument for a
registry over the ctx.query a module already holds: a copy of any of those living
outside engagement/ would drift the first time core improved the original, and the
drift would surface as a mail somebody already received.

The two halves behave differently, deliberately:

  - Templates re-ensure on every boot, so a bumped seedVersion reaches every
    deployment except the ones where an operator edited that row.
  - Rule groups are ONE-SHOT, each under its own settings guard — re-ensuring
    would resurrect a rule an operator deleted and reset one they enabled. This is
    11a's seed-key finding stated as an API rather than as a warning: a rule
    appended to an existing group reaches fresh installs only, and one that must
    reach stamped deployments takes a new group key.

Three prohibitions, each a shipped mistake that would only surface as mail: a
seeded rule is always enabled = 0 (Q3's invariant, ignored rather than refused so
a typo cannot take a module offline at boot); a module may not mark a template
protected; and a rule may only name its own trigger ids and its own or core's
template keys, with template keys namespaced because the key column is UNIQUE.

Runs from modules/lifecycle.js boot() rather than seedDefaults(), and that is
forced rather than chosen: server.js seeds before it requires app.js, and
requiring app.js is what runs the loader — at the moment core seeds, no module has
registered anything. Placed after the installed_modules reconcile (so a disabled
or failed module is skipped) and before the onBoot dispatch (so a module warming a
cache may assume its rules exist).

16 new tests; 1549 core tests green; check:modules clean.

Refs docs#/ENGAGEMENT.md Phase 11b decision 7.

Co-Authored-By: Claude <noreply@anthropic.com>

server/engagement-triggers.json

@@ -1,6 +1,6 @@
 {
   "_comment": "Generated event-trigger inventory - the authoritative freeze of CORE's engagement contract (docs/website/ENGAGEMENT.md 4.3). Regenerate with `npm run engagement:manifest` in website/server. A renamed variable, a changed type or a widened ceiling breaks stored templates and rules, so the diff here is the review signal. A module ships its own copy in its bundle; this file never contains one.",
-  "moduleApiVersion": "1.8.0",
+  "moduleApiVersion": "1.9.0",
   "triggers": [
     {
       "id": "news.post",