The naive LIFO tag stack pushed every start tag and popped on any end tag by
position, not name. Real email HTML almost always writes void elements
(<meta>, <br>, <img>, ...) without a self-closing slash -- e.g. <head><meta
charset=...><meta name=viewport ...></head> pops a "meta" frame for the
literal </head>, leaving skip_depth stuck at 1 for the rest of the document.
Caught live during the faza-mailowa Etap A dry-run spot-check (plan §7,
Krok 4 calibration step) against real archived mail: a genuine HTML-only
promotional email extracted to '' entirely. Re-running the Etap A dry-run
after the fix dropped body_empty from 3253/13300 (24.5%) to 291/13300
(2.2%) -- the bug was silently discarding real content from a meaningful
slice of HTML-only mail.
Fix: void elements are never pushed onto the stack (so they can never
desync it); every other closing tag searches backward for its matching
open tag and truncates the stack from there, which also self-heals other
malformed nesting instead of only the void-tag case. 3 new regression
tests reproduce the exact <head><meta>...</head> pattern, br/hr line
breaks, and a stray unmatched closing tag.
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