fix: resolve all synq_clickhouse and synq_postgres corpus failures

[lustefaniak]

Summary

• Fixes all 52 corpus failures in synq_clickhouse (39) and synq_postgres (13) categories
• No regressions — all existing tests pass, overall corpus improved from 894 to 840 failures

ClickHouse additions

• ALTER TABLE ... MODIFY COLUMN (with optional data type and REMOVE DEFAULT)
• ALTER TABLE ... MATERIALIZE COLUMN/PROJECTION
• ALTER TABLE ... DROP INDEX [IF EXISTS]
• ALTER TABLE ... UPDATE ... WHERE (lightweight mutations)
• EXCHANGE TABLES t1 AND t2 statement
• CREATE TABLE t1 AS t2 ENGINE = ... syntax
• ADD PROJECTION IF NOT EXISTS (IF NOT EXISTS after PROJECTION keyword)

PostgreSQL additions

• DO $$ ... $$ anonymous code blocks
• ALTER TABLE ... UPDATE (ClickHouse syntax tolerance)
• jsonb ? (has key) binary operator
• FOREIGN KEY REFERENCES without column list in table constraints
• DROP COLUMN / ADD COLUMN tolerance for missing identifiers/types

Keywords added

EXCHANGE, MATERIALIZE, MODIFY, REMOVE

[github-actions]

Corpus Parsing Report

Total: 104246 passed, 840 failed (99.2% pass rate)

Changes

✅ 2 improvement(s) (now passing)
➕ 2106 new test(s)

By Dialect

Dialect	Passed	Failed	Total	Pass Rate	Delta
ansi	583	129	712	81.9%	+2
bigquery	35193	86	35279	99.8%	-
clickhouse	3908	93	4001	97.7%	+793
databricks	1361	31	1392	97.8%	-
duckdb	712	63	775	91.9%	-
hive	32	29	61	52.5%	-
mysql	150	69	219	68.5%	-
postgres	1502	66	1568	95.8%	+1313
redshift	13449	59	13508	99.6%	-
snowflake	47232	195	47427	99.6%	-
sqlite	44	16	60	73.3%	-
trino	80	4	84	95.2%	-

✅ Improvements (2)

sqlglot_ansi/alter/1e12975cc239.sql
sqlglot_ansi/alter/912d29d69810.sql

➕ New Tests (2106)

synq_clickhouse/_blocks/001156589b0b.sql
synq_clickhouse/_blocks/017d8ea38806.sql
synq_clickhouse/_blocks/01b9e98a1db7.sql
synq_clickhouse/_blocks/01dea59ad2e1.sql
synq_clickhouse/_blocks/02c50a9e9614.sql
synq_clickhouse/_blocks/03aece7136f9.sql
synq_clickhouse/_blocks/047521f9ff79.sql
synq_clickhouse/_blocks/05ee5cb42acb.sql
synq_clickhouse/_blocks/065e66bb6f9e.sql
synq_clickhouse/_blocks/09c754a80936.sql
... and 2096 more