feat: implement CHECK constraints

[RichardKnop]

CHECK constraints are fully implemented. Here's what was added:

Parser (internal/parser/):

• New step stepCreateTableColumnCheck inserted after stepCreateTableColumnDefaultValue in the CREATE TABLE state machine
• "CHECK" added to reservedWords
• stepCreateTableColumnUnique when UNIQUE is found now also flows through stepCreateTableColumnDefaultValue (allowing DEFAULT+CHECK after UNIQUE)
• Raw expression text captured via p.sql[startPos:p.i] — stored as col.Check; parsed tree stored as col.CheckCond

Data model (internal/minisql/stmt.go):

• Column gains Check string and CheckCond *ConditionNode
• createTableDDL() emits check (expr) so constraints survive WAL/schema round-trips

Validation (internal/minisql/check.go):

• validateCheckConstraints() iterates columns, calls row.CheckOneOrMore(col.CheckCond.ToDNF())
• ErrCheckConstraintViolation{ColumnName, Expr} returned on first failure
• Called from insert.go after row assembly and cursor.update() after updates are applied

[github-actions]

Code Coverage

✅ Total: 70.3% (threshold: 70%)

Package	Coverage
github.com/RichardKnop/minisql	22.2%
github.com/RichardKnop/minisql/e2e_tests	[no
github.com/RichardKnop/minisql/internal/minisql	70.8%
github.com/RichardKnop/minisql/internal/parser	84.8%
github.com/RichardKnop/minisql/pkg/bitwise	100.0%
github.com/RichardKnop/minisql/pkg/lrucache	81.7%