Respect home path boundaries in pwd

[aiirvizionz]

Summary

• avoid shortening sibling paths that merely share the home path prefix
• keep exact home and true child paths rendered with ~`n- add focused tests for exact home, child paths, and same-prefix sibling paths

Tests

• npm.cmd test
• node --check src\pwd.mjs
• node --check test\pwd.test.mjs
• git diff --check