composer: lossless long-syntax import/export, long-syntax-aware valid…

[casablanque-code]

…ation

Import/export:

• ports/volumes lists mixing short and long syntax are split instead of parked wholesale in Extra; ExportYAML merges both back into one list. Previously adding a short entry through the form to a service that had long-syntax entries emitted the 'ports'/'volumes' key twice.
• A build set through the form now replaces a build mapping kept from import (was a duplicate 'build' key); same rule for healthcheck.
• healthcheck: add start_period/start_interval (were silently dropped on round trip). Blocks with disable:, no test, a string test or unknown keys are kept verbatim instead of being narrowed to HealthCheck.
• environment mapping form: 'FOO: ""' now becomes 'FOO=' (empty string) and only a null value becomes pass-through 'FOO'.
• Unmodeled keys are exported in sorted order, so saving is deterministic.

Validation (own semantic checks now also see what lives in Extra):

• build mapping without image no longer errors with 'image or build required'.
• image together with build is no longer an error: it is valid per the Compose Specification (image names the built image).
• service_healthy, 'database has no healthcheck' and 'database has no volumes' consider healthchecks/volumes kept in Extra.
• docker.sock mounts and all-interfaces ports are detected in long syntax too.
• validate start_period / start_interval durations.

New views on ServiceConfig: HasBuild, HasHealthCheck, HasVolumes, PortStrings, VolumeStrings.

What changed

How to test

Checklist

• make lint / npm run lint passes
• make test / npm test passes
• Documentation updated (if needed)
• Commits follow Conventional Commits