fix: substring window is wrong for start positions below 1

[spokodev]

Problem

substring(str from S for L) selects the window [S, S+L) clipped to valid (>= 1) positions. When S is below 1, pg-mem applies the length from the clamped start (position 1) instead of the original S, so it returns too many characters:

select substring('012345678' from 0 for 3);   -- pg-mem: '012', postgres: '01'
select substring('012345678' from -1 for 3);  -- pg-mem: '012', postgres: '0'
select substr('012345678', 0, 3);              -- pg-mem: '012', postgres: '01'

The same sqlSubstring helper backs the substring/substr functions and the substring(... from ... for ...) form.

Ref: Postgres string functions

Cause

sqlSubstring clamped the start to 0 but still passed the full length to substr(start, len), so the window's end shifted right by the amount that was clamped off the front.

Fix

Compute the window end from the original start (end = from + len), clamp only the start, and return an empty string when the window ends at or before the start. Also default an omitted FROM to position 1 (so substring(x for n) is unchanged) and update overlay(), which passed from = 0 relying on the old clamp. Tests added in simple-queries.spec.ts.