feat(build): invoke zeta-makepkg with conditional --force

- Add tofu.build module with runMakepkg() public API
- BuildException for missing binary, non-zero exit, missing output
- Real-time stdout/stderr streaming with last-20-lines stderr capture
- Conditional --force flag based on force parameter
- zetaToolchainPath resolution from Config
- 7 unittests using fake zeta-makepkg shell scripts
- pipeProcess + Redirect.stderr for tee approach

.omo/evidence/task-14-tofu-core.log

@@ -0,0 +1,15 @@
+=== tofu build task 14 — evidence log ===
+
+=== dub test ===
+     Warning warningsAsErrors: Use "buildRequirements" to control the warning level
+    Building tofu ~main: building configuration [tofu-test-application]
+installing package...error: build failedreason: missing dependencytestpkg-2.1 is already installed -- use -ReProvide to reinstall13 modules passed unittests
+
+=== dub build ===
+     Warning warningsAsErrors: Use "buildRequirements" to control the warning level
+    Building tofu ~main: building configuration [application]
+     Linking tofu
+    Finished To force a rebuild of up-to-date targets, run again with --force
+
+=== file listing ===
+429 src/tofu/build.d