Own the stamp extraction in-house; propagate the coadd centroid (drop sf_tools) #810

Resolves Axel's condition on #767 (options 2+3): reimplement the postage-stamp extraction in ShapePipe and propagate the coadd centroid at the same time, so the wcs centroid prior is computed in exactly the same way as the stamp extraction — by construction, not by after-the-fact matching.

What changes

• vignetmaker gains a shapepipe-owned get_stamps() (zero-pad by radius, slice around round(pos)), replacing sf_tools.image.stamp.FetchStamps. Bit-identical to FetchStamps for in-bounds objects; an out-of-bounds centre now raises instead of being silently wrapped modulo the image shape.
• The extractor records, per object per epoch, the rounded extraction pixel and the sub-pixel offset, carried on the multi-epoch vignette (no sidecar). ngmix's wcs (coadd) centroid consumes that stored offset instead of re-projecting the sky position through galsim and re-rounding — so a milli-pixel WCS disagreement can no longer flip a rounding tie and put the centroid prior a full pixel off.
• CENTROID_SOURCE hsm|wcs stays selectable. sf_tools is dropped from the dependencies.

Out of scope: the real-data hsm-vs-wcs A/B (needs a real-data run), and removing the HSM path (the open question on #767).

— Claude on behalf of Cail