Add host-path alias symlink startup hook and validation coverage

[UltraBob]

Summary

Implements host-path alias parity via a startup symlink strategy instead of a bind-mount alias.

What changed

• Added .ddev/web-entrypoint.d/90-dcq-host-path-alias.sh.
• Hook now creates a host-path alias symlink to /var/www/html at container startup.
• Added opt-out via DCQ_HOST_PATH_ALIAS=0|false|off.
• Added cleanup behavior for add-on-managed aliases when disabled.
• Wired new entrypoint hook into install.yaml project_files and removal_actions.
• Documented behavior and env var in README.md.
• Added bats coverage for:
  • existing path-map behavior,
  • alias creation behavior,
  • alias-disable behavior.

Important fix included

Entry-point scripts are sourced by DDEV. The initial implementation used exit, which could terminate container startup when aliasing was disabled. This PR fixes that by using safe sourced-script return behavior.

Validation

Targeted bats checks pass:

• path map prefers DDEV_HOST_PROJECT_ROOT
• host-path alias symlink is created on restart
• host-path alias can be disabled via DCQ_HOST_PATH_ALIAS

Notes

This PR establishes the alias architecture and keeps existing path-mapping wrappers in place for safety.
A follow-up phase will investigate removing/simplifying wrapper path-mapping logic to rely on host-path parity directly.

Refs #19

[UltraBob]

Superseded by PR #21 to avoid backlog-specific branch naming. Please review #21.