server: narrow monotonicity properties and refresh the corner sample #150

The manager accepted a726199 with these non-blocking follow-ups.

• server/tests/test_more_space.py asserts PASS < UNSURE < FAIL never worsens when an obstacle moves away. Its strict xfail demonstrates that this statement is too strong: removing an uncertain route obstruction can reduce the length error from 1.1 to 0.1 ft, correctly resolving an overlong route from UNSURE to FAIL. Narrow the property: a PASS never gets worse; a FAIL becomes PASS only when its margin exceeds its error; an UNSURE may resolve either way. Keep an isolated regression for the legitimate uncertainty reduction. The supplementary measured-margin assertion currently allows a small negative margin down to -EPS; make its statement match the dedicated strict equality tests.
• The past-limit-end repair changes the selected spot in review-corner-not-walked.json from [1.458,4.042] to [1.25,3.833], still manual_review. The captured ground past the left limit now counts, leaving only the unexplored-right-end request. Refresh web/src/samples/corner-not-walked/result.json in web: placement review page #15 with that owner rather than editing it from the server branch.

These do not establish a false PASS. Source and deployment changes are not requested by this follow-up.