fix(window): give RANGE frames peer-group semantics (P24)

[TimelordUK]

A RANGE frame was evaluated as ROWS. get_frame_rows carried the comment "RANGE frame - based on ORDER BY values (not yet fully implemented) / For now, treat like ROWS" above a verbatim copy of the ROWS arm.

RANGE is defined over peer groups - all rows tying on every ORDER BY key - where ROWS counts physical rows. On distinct keys the two coincide, so this only shows up against ties, which is why it survived until null_edges.csv existed. With SUM(score) OVER (ORDER BY score) over scores 10,20,30,50,50,... the first 50 saw 110 instead of 160: its own peer excluded from its own frame.

The damaging half is the default frame. SUM(x) OVER (ORDER BY y) with no explicit frame means RANGE UNBOUNDED PRECEDING .. CURRENT ROW, and is the common way to write a running total - silently wrong wherever y has duplicates.

Both halves were one defect. The parser already synthesised the correct default frame, so the entire divergence lived in the evaluator and one fix closed both corpus cases.

• OrderedPartition gains peer_bounds, computed in a single linear pass over the already-sorted rows, so all-equal keys cost no more than all-distinct.
• CURRENT ROW resolves to the first row of the peer group as a start bound and the last as an end bound. That asymmetry is the whole mechanism, so win_range_frame_peer_start covers the start side the original cases missed.
• Sorting and peer detection must agree exactly or frames land mid-group; both now route through one compare_by_sort_cols, and peers are the rows it calls Equal. With no ORDER BY that makes the partition a single peer group, which is what the standard specifies, with no special case.
• RANGE with a numeric offset is value-based, not positional, and is now rejected at WindowContext construction instead of silently returning the ROWS answer. Filed as P33; ROWS offsets are untouched.

Parity 129 -> 133 AGREE (+2 fixed, +2 new coverage), GAP 13 -> 14 for P33. Explicit ROWS frames verified unchanged. Four unit tests cover peer groups from both bound sides, the ROWS contrast, and the offset rejection.