Bound DETAILS_TAG's Opener Scan So an Unclosed Opener Costs Linear Time #2386

Found by a local-strict-review pass while working #2384 on feature/auto-2384. It predates that branch, whose opener text is unchanged from develop.

DETAILS_TAG's opener alternative in scripts/pr_review.py, <details(?:[ \t\n\f\r][^>]*|/)?>, has an unbounded [^>]* that crosses lines. For each <details with no > after it, the scan runs to the end of the text, so a body made of such lines costs quadratic time in every reader that pairs tags (details_spans, mask_narrative, and through them marker_blocks and unrecognized_in).

A constructed body of "<details x\n" * n measured about 0.06s at 11k characters, 0.26s at 22k, 1.05s at 44k, and 2.3s at 66k per reader call. GitHub caps a body at 65536 characters, and status runs several readers per body.

Done looks like: the opener is bounded so the scan is linear, for example by stopping its tail at < or at a blank line, with a timing test or a bounded-input test built from constructed text, and the existing tag tests still passing.