Refactor: CREATE TABLE / ALTER TABLE grammar and AlterExpression model

Grammar: extract shared helper productions to eliminate redundancy
- ReferentialActionsOnIndex: ON DELETE/UPDATE action (was duplicated 4x)
- CheckConstraintSpec: CHECK (expr) (was duplicated 3x)
- ForeignKeySpec: FOREIGN KEY ... REFERENCES ... (used by both CREATE/ALTER)
- AlterExpressionUsingIndex: USING [INDEX] name (was duplicated 4x)
- AlterExpressionConstraintTail: constraint state + USING + COMMENT tail
- AlterExpressionColumnChanges: DROP/SET DEFAULT, SET VISIBLE/INVISIBLE,
  bracketed multi-column (was duplicated 2x)
- AlterExpressionDiscardOrImport: merged identical DISCARD/IMPORT blocks

Grammar: decompose monolithic productions
- CreateTableConstraint: all constraint types inside CREATE TABLE (...)
  CreateTable shrinks from 211 to 86 lines
- AlterExpressionAddConstraint: CONSTRAINT clause in ALTER TABLE
- AlterExpressionDrop: all DROP variants
- AlterExpressionPartitionOp: 11 partition maintenance operations
  AlterExpression() shrinks from 754 to ~430 lines

AlterExpression.java: unify FK handling
- Standalone ADD FOREIGN KEY now uses ForeignKeySpec/ForeignKeyIndex,
  same as CONSTRAINT ... FOREIGN KEY path
- Deprecated fkColumns/fkSourceTable/fkSourceSchema/fkSourceColumns
  fields and associated methods in favor of ForeignKeyIndex via setIndex()
- Deprecated referentialActions methods on AlterExpression
- Fields still populated for backward compatibility; rendering delegates
  to ForeignKeyIndex.toString() to preserve referential action order

AlterExpression.java: split toString() into focused methods
- toStringConstraintAlter, toStringAlterColumn, toStringSimpleKeyword,
  toStringRename, toStringDropSpecial, toStringConvert, toStringPartition,
  toStringGeneral — replacing a 330-line if/else chain

No breaking changes to public API. All existing tests pass.

Signed-off-by: Andreas Reichel <andreas@manticore-projects.com>
Signed-off-by: manticore-projects <andreas@manticore-projects.com>

Author: manticore-projects