Reject invalid zero-width SV candidate intervals

[iamh2o]

Summary

• reject invalid A/B inclusive intervals before coverage lookup
• guard get_region against non-positive widths
• emit a deterministic diagnostic with exact bounds and support
• do not swap bounds or fabricate coverage

Motivation

A real LongRead/short-read SV analysis exposed a candidate whose normalized interval had invalid inclusive bounds. The current coverage expression divides by end-start+1, causing ZeroDivisionError. This patch rejects the structurally invalid candidate before coverage calculation and keeps a defensive guard in get_region.

Validation

• native Linux arm64 compiled suite: 6/6 passed
• emulated Linux amd64 compiled suite: 6/6 passed
• installed amd64 wheel smoke test passed

The branch is based on TIDDIT 3.9.6 (141e39f).

[J35P312]

Hello!
Thanks for this pull request, it looks really nice! I hope you get a bugfree tiddit experience now ;).

Best regards
Jesper