Add docs/SAM.md — a user-facing reference for the regex deviation, with
examples verified against the engine:
- leftmost-greedy (PCRE) vs leftmost-longest (sam): diverges only on
overlapping alternation (a|ab); order alternatives longest-first.
- ^/$ anchor buffer-wide, not per-line, by default. Workarounds:
structural ,x/.+/ … (preferred) or the (?m) inline flag.
- identical behaviour table (. and newlines, empty-match advance, &/\N).
- PCRE bonuses sam lacks: \d \w \b, lookahead, non-greedy, (?i)(?m)(?s).
Cross-link from PLAN.md and surface the two biggest gotchas (buffer-wide
anchors, alternation order) directly in the Sam panel hint label.
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