Quantize coordinates on save (#20)

[MisterGC]

Fixes #20 — auto-save now writes integer-pixel coordinates so a 1-px drag produces a 1-line diff instead of ~14 digits of float noise.

• grafli/format.py: single _q() helper rounds box / note / image x/y/w/h and arrow @dx,dy label offsets on serialize; replaces the per-field "emit int when it happens to equal int(v)" checks. Drops arrow offset entirely when both components round to 0.
• Tests: updated the two existing tests that asserted fractional preservation; added six new tests (box/note/image/arrow quantization, the (0,0) offset omission, no-float-noise on the showcase round-trip, and a parse→serialize→parse→serialize byte-stable double round-trip). 332 / 332 passing.
• examples/*.grafli: re-saved through the new serializer to remove the float noise already present in the repo (4 files; ~83 lines re-saved per file). Legitimate decimals inside labels (v0.1.0, 2.0) are untouched.

Test plan

• pytest -q — 332 passed
• Drag any box in examples/showcase.grafli and save; git diff should show a 1-line integer-coord change with no float noise
• Open / save each example file untouched — round-trip leaves it byte-identical