fix(rtt): Down.mode() uses self.flags instead of self.mode

[by965738071]

• Bug: Down.mode() referenced self.mode (the method itself) instead of the flags field, causing a compile error on Zig 0.17
• Fix: changed self.mode to self.flags, matching the Up.mode() pattern
• Add build.zig test step with 5 unit tests covering mode get/set, Writer/Reader compilation, and Up mode verification
• Add test for Down set_mode + mode round-trip

[mattnite]

How did you test this?

[by965738071]

The Down.mode() method was dead code — it's never called from outside the module, which is why the bug went unnoticed. Zig only instantiates method bodies when they're used, so the compiler never type-checked it.

I verified the fix by:

1. Confirming the pattern matches Up.mode() which uses self.flags & 3
2. Adding unit tests that exercise Down.mode() and set_mode() round-trip