…che#1577)

Enable bare graph patterns as boolean expressions in WHERE clauses:

  MATCH (a:Person), (b:Person)
  WHERE (a)-[:KNOWS]->(b)        -- now valid, equivalent to EXISTS(...)
  RETURN a.name, b.name

Previously, this required wrapping in EXISTS():
  WHERE EXISTS((a)-[:KNOWS]->(b))

The bare pattern syntax is standard openCypher and is used extensively
in Neo4j.  Its absence was the most frequently cited migration blocker.

Implementation approach:
- Switch the Cypher parser from LALR(1) to Bison GLR mode.  GLR handles
  the inherent ambiguity between parenthesized expressions '(' expr ')'
  and graph path nodes '(' var_name label_opt props ')' by forking the
  parse stack and discarding the failing path.
- Add anonymous_path as an expr_atom alternative with %dprec 1 (lower
  priority than expression path at %dprec 2).  The action wraps the
  pattern in a cypher_sub_pattern + EXISTS SubLink, reusing the same
  transform_cypher_sub_pattern() machinery as explicit EXISTS().
- Extract make_exists_pattern_sublink() helper shared by both
  EXISTS(pattern) and bare pattern rules.
- Fix YYLLOC_DEFAULT to use YYRHSLOC() for GLR compatibility.
- %dprec annotations on expr_var/var_name_opt resolve the reduce/reduce
  conflict between expression variables and pattern node variables.

Conflict budget: 7 shift/reduce (path extension vs arithmetic on -/<),
3 reduce/reduce (expr_var vs var_name_opt on )/}/=).  All are expected
and handled correctly by GLR forking + %dprec disambiguation.

All 32 regression tests pass (31 existing + 1 new).  New
pattern_expression test covers: bare patterns, NOT patterns, labeled
nodes, AND/OR combinations, left-directed patterns, anonymous nodes,
multi-hop patterns, EXISTS() backward compatibility, and non-pattern
expression regression checks.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

1. Move "Helper function to create an ExplainStmt node" comment from
   above make_exists_pattern_sublink() to above make_explain_stmt()
   where it belongs.

2. Add block comment documenting the %expect/%expect-rr conflict
   budget: 7 S/R from path vs arithmetic on - and <, 3 R/R from
   expr_var vs var_name_opt on ) } =.

3. Clarify test comment: "Regular expressions" -> "Regular (non-pattern)
   expressions" to avoid confusion with regex.

Regression test: pattern_expression OK.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- Pattern expressions are now accepted anywhere an expr is valid
  (RETURN, WITH, SET, CASE, boolean combinations), not only WHERE.
  This matches openCypher semantics and documents the broader surface
  area that was already implicitly enabled by adding anonymous_path
  to expr_atom.  Added regression tests for each new context:
  RETURN projection (bare and AS-aliased), mixed with other
  projections, CASE WHEN, boolean AND/OR combinators, SET to
  persist a computed boolean property, and WITH ... WHERE pipeline.

- Remove the hardcoded `%expect 7` / `%expect-rr 3` conflict budget
  from cypher_gram.y.  The exact conflict counts can drift across
  Bison versions and distros, which would break builds even though
  the grammar is correct (GLR handles the conflicts at runtime via
  fork + %dprec).  Instead, pass -Wno-conflicts-sr / -Wno-conflicts-rr
  via BISONFLAGS in the Makefile so the build stays clean without
  binding us to a specific Bison release.  Kept a block comment in
  the grammar explaining why GLR conflicts are expected and how
  they resolve.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>