Support service extends in Compose files

[GtechGovind]

I added service inheritance resolution before typed Compose decoding, so a service can inherit its image or build configuration from a service in the same file or another Compose file.

The resolver follows chained references, detects missing files or services and circular references, and applies field-specific merge behavior for environment values, ports, commands, volumes, and env files. Paths inherited from an external file remain anchored to that file. Only referenced service configuration is inherited; unrelated external services and resources are not imported.

I added static regression tests for the reported inherited-image failure, same-file and external references, shorthand and chained inheritance, merge precedence, external paths, YAML anchors, missing references, and cycles.

Validation:

• swift test --filter ComposeExtendsTests: 11 passed
• swift test --filter Container_Compose_StaticTests: 260 passed
• swift build --build-tests: passed
• make build: passed

The dynamic container tests were not run; this change is covered by the static parsing suite.

Addresses #7.