fix: keep y0/y1 in sync when clipping objects in clip_obj

[otiscuilei]

Who hits this

Anyone reading y0/y1 off an object that was clipped by .crop(...), .within_bbox(...), or .outside_bbox(...). Those fields are part of the public per-object schema (exported via to_dict, .objects, and the CSV/JSON output, documented as "distance from bottom of page"), so consumers that read y0/y1 — or compute geometry/height from them — on a cropped page get silently wrong numbers. Text extraction is unaffected (it uses top/doctop/x0), so this is silent corruption of the coordinate fields rather than a crash.

Root cause

clip_obj in pdfplumber/utils/geometry.py rewrites an object's top-based coordinates (x0, x1, top, bottom, doctop, width, height) but never updates the PDF-space y0/y1. pdfplumber objects carry the same box in two coordinate systems with the invariant top == page_bottom_ref - y1 and bottom == page_bottom_ref - y0. After a vertical clip, top/bottom shift but y0/y1 keep their pre-crop values, so the two representations describe different boxes and y1 - y0 no longer equals height. The sibling helpers move_object and resize_object both already update y0/y1 when present; only clip_obj was missing it.

The fix

After computing the clipped top/bottom, also update y0/y1 when present, mirroring resize_object: y1 tracks top, y0 tracks bottom.

if "y1" in copy:
    copy["y1"] = obj["y1"] - diff
if "y0" in copy:
    copy["y0"] = obj["y0"] - (dims["bottom"] - obj["bottom"])

Before / after

Cropping a page so chars[0] (top=117.18, bottom=135.18, y0=656.82, y1=674.82, height=18) is cut in half vertically:

• Before: top=126.18, bottom=135.18, height=9.0, but y0=656.82, y1=674.82 -> y1-y0 = 18.0 (stale); page.height - y1 = 117.18 != top
• After: y1=665.82, y0=656.82 -> y1-y0 = 9.0 = height, and page.height - y1 = 126.18 = top, page.height - y0 = 135.18 = bottom

Notes

• Adds a regression test test_crop_preserves_y0_y1 in tests/test_basics.py that crops through a char vertically and asserts y1-y0 == height and the top/bottom to y0/y1 invariant. It fails on the current code and passes with this change.
• Updates CHANGELOG.md (Unreleased / Fixed) and adds a contributor entry to README.md.
• make lint passes (flake8, mypy, black, isort).