fix: root cwd-aware generation paths

[tiborvass]

What changed

• Anchor workspace-root-relative Java module paths before resolving them through the cwd-aware polyfill.
• Add an end-to-end regression check for generation from both a module's parent directory and the module root.

Why

After the cwd-aware discovery change in #8, Mod.rootPath remained workspace-root-relative, but moduleSource interpreted an unanchored path relative to Workspace.cwd. Running generation below the workspace root therefore duplicated the cwd, for example .dagger/modules/.dagger/modules/hello, and failed to resolve the module.

Prefixing rootPath with / at the polyfill boundary preserves it as the stable workspace identity while leaving the rest of generation's workspace-relative staging unchanged.

Validation

• Reproduced the regression before the fix with dagger --x-release=v1.0.0-beta.7 check e-2-e:generate-cwd-check.
• Focused regression check passes after the fix.
• Full beta.7 suite passes: 8/8 checks.