fix: refactor settings rendering into settingsgradle

[cdsap]

Summary

Problem

ProjectWriter.kt:88-105 both assembles Gradle include syntax and writes settings files. SettingsGradle.kt already owns settings-file rendering, creating a split boundary and coupling file orchestration to Gradle syntax.

Why this matters

The settings format is harder to unit-test independently, and future changes can require modifying both the writer and root-project generator.

Proposed change

Move module include rendering into SettingsGradle, passing mapped module paths or module data from ProjectWriter. Keep ProjectWriter responsible only for writing the resulting content to each language-specific settings file.

Notes

This keeps root-project document generation in the generator layer and leaves filesystem writes in the writer layer, clarifying the infrastructure boundary without introducing a broader architectural change.

Fixes #475

Changes

• project-generator/src/main/kotlin/io/github/cdsap/projectgenerator/generator/rootproject/SettingsGradle.kt
• project-generator/src/main/kotlin/io/github/cdsap/projectgenerator/writer/ProjectWriter.kt
• project-generator/src/test/kotlin/io/github/cdsap/projectgenerator/generator/rootproject/SettingsGradleTest.kt

Verification

• ./gradlew :project-generator:unitTest
• ./gradlew :cli:test
• ./gradlew ktlintCheck