abaco: pageselect1 ordering — same-line drag orders by X

Addresses the NOTES entry "fix text selection" on the click-drag path.

pageselect1 at page.c:541 decides p->top / p->bot ordering from a
strict-less-than comparison on raw pixel Y values.  Mouse hardware
emits 1-2 pixel Y wobble during a drag.  Within a single visual
text line that wobble flips opos.y vs npos.y between events, so
p->top / p->bot flip between canonical and reversed multiple times
per drag.  In the reversed state (p->top to the right of p->bot on
same row, or diagonally offset on different rows), istextsel +
html.c drawing produces a two-piece highlight: from the start of
the row to p->bot, nothing in the middle, from p->top to the end
of the row — the "complement" symptom a user sees as a broken
highlight on in-line drags.

Multi-line drags aren't affected: different Lines occupy
non-overlapping Y ranges, so pixel-Y comparison is stable across
1-2 px wobble.  Only same-line drags trip on the wobble.

Fix: if both points resolve to the same Line (via linewhich, which
pagedoubleclick already uses at page.c:583), order by X.  Otherwise
fall back to the existing pixel-Y comparison.

Minimal fix: one if-else becomes one if-elseif-else, no new
variables, linewhich inlined.  Three lines added net.

Also drops the NOTES "fix text selection and double clicking"
bullet — abaco-doubleclick-typo (bracket/quote) and
abaco-pagedoubleclick-guard (nil-deref past EOL) cover the other
two code issues the bullet referenced.

RFC relevance: none

--- sys/src/cmd/abaco/NOTES
+++ sys/src/cmd/abaco/NOTES
@@ -1,6 +1,5 @@
 Bugs:
-	* fix text selection and double clicking;

 Not Bugs:
 	* complaints like "gif: decode <stdin> failed: ReadGIF: can't recognize format ��"
 	  are caused by sites that return a jpeg and send Content-Type: image/gif.
--- sys/src/cmd/abaco/page.c
+++ sys/src/cmd/abaco/page.c
@@ -538,7 +538,14 @@

 		scrled = pagescrollxy(p, x, y);
 		npos = getpt(p, mp);
-		if(opos.y <  npos.y){
+		if(linewhich(p->lay, opos) == linewhich(p->lay, npos)){
+			/* same visual line — order by X (pixel-Y is
+			 * unstable against 1-2 px mouse-hardware wobble) */
+			if(opos.x <= npos.x)
+				p->top = opos, p->bot = npos;
+			else
+				p->top = npos, p->bot = opos;
+		}else if(opos.y < npos.y){
 			p->top = opos;
 			p->bot = npos;
 		}else{