fix(compose): preserve project env vars when compose specifies empty passthrough placeholders - #616

…n step

Review follow-ups on the oblien#614 fix.

- Move the layering to `compose/service-env-layers.ts` and give the rule a
  name (`inlineEmptyDefers`). Two surfaces spelled it out by hand; they now
  import it.
- Log every substitution. The service Env tab and the deploy wizard both go
  on showing the empty value the merge ignored, so the deploy log was the
  only surface that could explain the container — and it said nothing. Names
  only, never values, matching `resolveEnvPublicUrls` just below it.
- `describeRestorePlan` built its per-service overrides as a raw spread of
  the service row, under a comment asserting it matched the deploy merge. It
  no longer did, so the rollback confirm dialog reported a `frozen-wins`
  revert plus a `scopeAmbiguous` warning for every compose passthrough key
  the rollback leaves alone. It applies the same deferral now.
- Correct the docstring, which still stated the old flat precedence, and
  write down what the rule costs: an authored empty no longer clears a
  configured value, and an empty service-scoped row is the escape hatch.
- Tests for the frozen layer (the combination every non-rollback deploy
  has), the rollback replay, the accepted trade-off, the escape hatch, and
  the gap a value-shaped rule cannot close — `postgres://${U}:${P}@db/${D}`
  resolves to non-empty garbage and still wins.
- Note in `getAppConnectionView` that it is NOT the deploy merge: it omits
  the project layer entirely. Adding it changes what every existing app
  connection resolves to, so that is left for its own change.