Add the SQL renderer, AST dump, round-trip property and snapshots

The second half of PLAN.md's answer to "SQLite has no parse-tree oracle".
Accept/reject conformance cannot see a dropped clause or an operator
associated the wrong way, so two layers stand in for upstream goldens:

- ast.String/ast.Statements render a tree back to SQL. Not a formatter,
  and no promises beyond re-parseability. Parenthesisation needs no
  precedence logic, because the parser keeps explicit parentheses as
  ParenExpr nodes.
- internal/dump renders a node reflectively, so it cannot fall behind the
  node set: a field added to a node shows up in the next snapshot diff
  whether or not anyone remembered. Two option sets — everything, for
  snapshots, and shape-only for comparisons, which drops the spans and
  Raw text a round trip is entitled to change.
- TestRoundTrip runs parse/render/parse over all 20,815 accepting corpus
  cases and compares the trees, then checks that rendering is idempotent.
- TestASTSnapshots covers a hand-written tour of the node set under
  parser/testdata/ast, with -update to rewrite the goldens.

Two things this found:

- IndexedColumn recorded that a COLLATE was present but not which
  collation, so "FOREIGN KEY(b COLLATE nocase DESC)" lost the name.
- Three nodes built their Span from a composite literal that also called
  the parse function for their last child, so the span ended before the
  child began. Go does not order those evaluations.

Enum types gained String methods, so a snapshot diff names what changed
instead of printing an integer.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JzBeCg7rjweVW3uGPg5G7T

Author: claude

CLAUDE.md

@@ -13,7 +13,8 @@ https://sqlite.org/lang.html.
 - **Never edit `parser/testdata/*.test` by hand.** Corpus files are produced
   by `cmd/regenerate-parse` from SQLite's own test suite plus a real SQLite
   build (the oracle). `*.metadata.json` sidecars are updated by tooling
-  (`go test ./parser -check-parse`), not by hand.
+  (`go test ./parser -check-parse`), not by hand. The hand-written snapshot
+  inputs under `parser/testdata/ast/` are the exception — see below.
 - Every nontrivial parse function carries a comment naming the `parse.y`
   rule(s) it implements.
 - Error messages must match SQLite's parser byte-for-byte
@@ -47,6 +48,20 @@ semantic, so meyer is permitted to accept such statements. The pattern list
 lives in `internal/testfile` (`syntaxFamily`) and can be extended without
 regenerating the corpus, because the corpus stores raw oracle output.
 
+## Tree shape
+
+Accept/reject conformance cannot see a dropped clause or a mis-associated
+operator, so two further checks stand in for the parse-tree goldens SQLite
+cannot produce:
+
+- **Round trip** (`TestRoundTrip`): every corpus case that parses is
+  rendered back to SQL with `ast.Statements`, re-parsed, and the two trees
+  compared structurally with `internal/dump`. Spans and `Raw` fields are
+  excluded — the renderer promises re-parseability and nothing else.
+- **Snapshots** (`TestASTSnapshots`): `parser/testdata/ast/*.sql` are
+  hand-written and meant to be edited; their `.tree` goldens are rewritten
+  with `go test ./parser -update` and reviewed in the diff.
+
 ## The loop
 
 ```sh

ast/ddl.go

@@ -148,12 +148,12 @@ func (n *TableConstraint) Children() []Node {
 //	eidlist ::= eidlist COMMA nm collate sortorder.
 type IndexedColumn struct {
 	Span
-	Name    *Ident    `json:"name"`
-	Collate bool      `json:"collate,omitempty"`
-	Order   SortOrder `json:"order,omitempty"`
+	Name      *Ident    `json:"name"`
+	Collation *Ident    `json:"collation,omitempty"`
+	Order     SortOrder `json:"order,omitempty"`
 }
 
-func (n *IndexedColumn) Children() []Node { return nodes(n.Name) }
+func (n *IndexedColumn) Children() []Node { return nodes(n.Name, n.Collation) }
 
 // ForeignKeyAction is the action of an ON DELETE / ON UPDATE clause.
 type ForeignKeyAction int