Add .PHONY declaration to Makefile

[mgrl39]

Add .PHONY to Makefile targets

Changes:

• Explicitly declares all, fmt, test, build, and other non-file targets as .PHONY in the Makefile.

Why?

• Ensures these targets always execute (even if a file/directory with the same name exists, e.g., ./clean).
• Follows GNU Make conventions (Phony Targets docs).
• Improves reliability and avoids potential edge cases in builds.

Impact:

• No functional changes—just better Makefile hygiene.