fix(build): allow user-supplied binaries

[harrisoncramer]

Fixes three issues introduced in #528:

• build_and_start would shut down the server permanently when a user provides their own binary via server.binary. The version check would detect a mismatch (especially in
  Nix where there's no .git dir), call shutdown(), then try build(true) which returns nil for user-provided binaries, leaving the server dead. Fixed by skipping the version check cycle entirely for user-provided binaries.
• build() returned nil instead of false when a user-provided binary path doesn't exist, so build_and_start would proceed to start a non-existent binary. Now returns false, and build_and_start checks for it.
• The Windows .exe extension was dropped when the binary name changed from bin to server. Restored platform-aware naming.