fix: guard FizEncoders construction when no encoder axis is present

[rocketmark]

Summary

get_tracking_frame() in mosys/f4.py unconditionally constructs
FizEncoders(focus, iris, zoom) at the end of every frame. When no FIZ
axis appears in the packet, all three values remain None and FizEncoders
raises ValueError: FizEncoders requires at least one of focus or iris or zoom,
crashing the parser.

Demonstration

BEFORE: F4PacketParser.get_tracking_frame() on any packet with no encoder axes
raises ValueError: FizEncoders requires at least one of focus or iris or zoom

AFTER:
returns a valid frame with lens_encoders unset (None)

Impact

Any F4 packet that omits FIZ encoder axes causes get_tracking_frame() to
raise, aborting frame parsing. Callers receive an unhandled exception rather
than a partial frame.

Change

mosys/f4.py: wrap the FizEncoders construction in a guard so it is only
executed when at least one of focus, iris, or zoom was decoded from the
packet. One-line change.

test_mosys_reader.py: add test_no_encoder_axes_does_not_crash — a unit test
using a hand-crafted minimal packet with no encoder axes. Verifies the parser
returns a valid frame with lens_encoders is None.

Found via

Hypothesis-based property testing against a synthetic F4 packet generator.
Packets containing only the minimum required axes (timecode + focal length)
triggered the crash immediately on the first generated example.

[rocketmark]

Force pushed to resolve the branch conflicts. The change here doesn't conflict with the other changes.