Hash pf_gkey instead of shipping the concatenated grain tuple

pf_gkey is the Perspective table index -- an opaque handle read only by
table.update() and table.remove(). It was the raw concat_ws of every grain
column, which on the 24-column grain of fc_osm_skinny_29 averaged 233
characters. Being unique per row by construction, it also defeated Arrow's
dictionary encoding, so it alone accounted for 65.6 MB of a 109 MB payload
-- more than the other 31 columns combined, all of which do dictionary
cleanly.

md5 of the same string keeps the determinism undo depends on (routes/log.js
recomputes the key through this same grainOf) and keeps collisions out of
reach at 128 bits, while fixing the width at 32 characters.

Measured on the live 285,685-row aggregate: payload 109.0 MB -> 54.2 MB,
all 285,685 keys still distinct.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

lib/sql_generator.js

@@ -34,11 +34,17 @@ function grainOf(colMeta) {
     // parts and chr(30) stands in for NULL, so ('a', NULL) cannot collide with
     // (NULL, 'a') and a NULL stays distinct from an empty string — a collision
     // would silently merge two groups into one indexed row.
-    const key = (pfx = '') => `concat_ws(chr(31), ${[
+    //
+    // md5 of that, rather than the concatenation itself, because the key is an
+    // opaque handle -- nothing reads it but table.update() and table.remove().
+    // The raw form averaged 233 chars on a 24-column grain and, being unique per
+    // row, defeated Arrow's dictionary encoding: 65.6 MB of a 109 MB payload,
+    // more than every other column combined. 128 bits keeps collisions unreachable.
+    const key = (pfx = '') => `md5(concat_ws(chr(31), ${[
         ...cols.map(c => `COALESCE(${pfx}${q(c)}::text, chr(30))`),
         `${pfx}pf_iter`,
         `${pfx}pf_logid::text`
-    ].join(', ')})`;
+    ].join(', ')}))`;
 
     const groupCols = (pfx = '') => [...cols.map(c => `${pfx}${q(c)}`), `${pfx}pf_iter`, `${pfx}pf_logid`];