Keep image position stable at minimum zoom

[infeng]

Reproduction

When wheel zoom reaches minScale, another zoom-out keeps the scale clamped but still changes translateX and translateY. The regression test reproduces the horizontal drift from 436.9px to 411.8px at a 0.95 minimum.

Root cause

The scale was clamped before rendering, but position updates still used the requested zoomSpeed rather than the scale delta that was actually applied.

Fix

• Calculate translation from the effective scale delta after min/max clamping.
• Add regression coverage for repeated off-center wheel zoom at minScale.

Verification

• npm run verify
• 42 Jest tests passed
• Chrome 149 demo check: after reaching the default 0.1 minimum, an extra wheel zoom left the complete image transform unchanged
• Public npm registry scan passed

Fixes #95