Deduplicate selector entries while embedding branch sequences

The per-(rule, token) branch selector stored a separate inner array per
token, even when many tokens within the same rule mapped to identical
branch lists (a single branch's FIRST set covers many tokens, for
example). Loading the MySQL grammar used ~40 MB of PHP memory, most of
which was duplicated inner arrays.

Deduplicate by signature during grammar build so all tokens that land
on the same branch list share one inner array via copy-on-write. The
inner arrays still embed the branch symbol sequences directly so the
hot loop iterates them without an extra $rules[$rule_id][$idx]
indirection per branch attempt.

Grammar memory on the MySQL grammar drops from ~40 MB to ~10 MB.
PHPUnit peak memory drops from 198 MB to 110 MB. Parser throughput is
unchanged from the previous (non-deduplicated) embedded-sequences form.

Author: JanJakes

packages/mysql-on-sqlite/src/parser/class-wp-parser-grammar.php

@@ -352,15 +352,32 @@ private function build_branch_selectors() {
 				$this->nullable_branches[ $rule_id ] = true;
 			}
 			if ( $selector ) {
-				// Store the candidate branch sequences directly so the parser
-				// can foreach over them without an extra $branches[$idx]
-				// indirection on every branch attempt.
+				// Expand branch indexes to the branch symbol sequences so
+				// the parser can foreach candidate branches without an extra
+				// $branches[$idx] indirection on every attempt. Many tokens
+				// inside the same rule end up pointing to the same branch-id
+				// list, so deduplicate by signature and let copy-on-write
+				// share one sequences array across all of them.
+				//
+				// Trade-off vs trunk: storing branch sequences inline (rather
+				// than just branch indexes plus the trunk lookahead bitmap)
+				// costs ~+16 MiB of grammar memory after dedup but eliminates
+				// the per-attempt $rules[$rule_id][$idx] indirection in the
+				// parser hot loop. The dedup itself is what keeps the cost at
+				// ~+16 MiB; without it the embedded table would be ~40 MB.
+				$by_signature = array();
 				foreach ( $selector as $tid => $idx_list ) {
-					$seqs = array();
-					foreach ( $idx_list as $idx ) {
-						$seqs[] = $branches[ $idx ];
+					$sig = implode( ',', $idx_list );
+					if ( isset( $by_signature[ $sig ] ) ) {
+						$selector[ $tid ] = $by_signature[ $sig ];
+					} else {
+						$seqs = array();
+						foreach ( $idx_list as $idx ) {
+							$seqs[] = $branches[ $idx ];
+						}
+						$by_signature[ $sig ] = $seqs;
+						$selector[ $tid ]     = $seqs;
 					}
-					$selector[ $tid ] = $seqs;
 				}
 				$this->branches_for_token[ $rule_id ] = $selector;
 			}