Keep Video VAE RGB output finite for non-finite values

[morluto]

Problem

h3_video_frames.rgb documents that decoded values are F32 in [0,1], but
the Video VAE output boundary currently clamps with two ordered comparisons:

if (value < 0.0f) value = 0.0f;
if (value > 1.0f) value = 1.0f;

Both comparisons are false for NaN, so a non-finite decoder value passes into
the returned RGB buffer unchanged. It can then propagate through tiled or
temporal blending and reach the final integer conversion.

Change

Clamp the decoded value with fminf(fmaxf(value, 0.0f), 1.0f). The standard
floating-point min/max behavior selects the finite operand when only one
operand is NaN, making the output mapping deterministic:

Decoder value	Previous output	New output
NaN	NaN	0.0f
-INFINITY	0.0f	0.0f
+INFINITY	1.0f	1.0f

Finite values already inside [0,1] are unchanged, and ordinary out-of-range
values retain the existing saturation behavior.

Regression coverage

The real Video VAE test now has a focused --nonfinite-output mode. It sends a
minimal 2x1x1 all-NaN latent through the public h3_video_vae_decode API and
checks that every returned RGB component is finite and within [0,1].

Without the new clamp, the NaN output fails that contract check.

Validation

• Strict Clang syntax checks pass for the production and regression-test files.
• A direct fminf/fmaxf probe returns 0.0 for NaN and negative infinity,
  and 1.0 for positive infinity.
• git diff --check passes.