Reject truncated PPM pixel data

[treeform]

Fixes #563.

Summary:

• Reject PPM payloads whose decoded pixel count does not match the declared dimensions.
• Reject partial P6 pixels before decoding.
• Reject invalid maxVal == 0 values.
• Add a regression test for the truncated P6 payload from the issue.

Tests:

• nim r --hints:off --warnings:off tests\test_ppm.nim
• git diff --check