Improve JOIN execution, FK enforcement and table alias support

[efremropelato]

Summary of Changes

1. Foreign Key — Critical Crash and Missing Enforcement

Files: src/core/xdb_fkey.c, src/core/xdb_fkey.h, src/core/xdb_crud.c

• Crash fixed: xdb_create_fkey was writing through an xdb_filter_t* pointer that was never allocated (NULL), causing an immediate SIGSEGV on any CREATE TABLE ... FOREIGN KEY. Added real storage (fkey_filters[]) in the xdb_fkeym_t struct.
• Collateral bug fixed: wrong shift (>>8 instead of >>3) in the field bitmap computation, which left memory uninitialized.
• Enforcement implemented (previously absent, only dead code under #if 0):
  • Existence check on INSERT/UPDATE: rejects with XDB_E_CONSTRAINT if the FK value does not exist in the referenced table.
  • RESTRICT constraint on DELETE: rejects deletion of the parent if child rows exist (also required fixing the reverse index fkeyref_objm, which was registered on the wrong table).
• Known limitation: RESTRICT only, not CASCADE/SET NULL (deferred to a later increment).

2. Multi-Table JOIN — From Completely Non-Functional to Working

Files: src/parser/xdb_parser_dml.c, src/core/xdb_crud.c

JOIN was dead code (#if 0) that was never completed. Distinct bugs fixed:

• ON clause parser was disabled → restored with multi-column support.
• Recursive query never executed in xdb_sql_join, wrong row indexing (pRowPtrs[level] instead of level-1).
• Wrong combined metadata layout (columns copied from the wrong offset → crash on output).
• NULL bitmap never merged across joined tables (NOT NULL columns appeared as NULL).
• Design choice: only "chained joins" supported (each table joins against the previous one); a non-chained JOIN is rejected with a clean error instead of silently producing wrong results.
• Known limitation: JOIN with VARCHAR/VARBINARY/JSON columns not supported (clean error, not a crash).

3. Table Alias and Self-Join

File: src/parser/xdb_parser_dml.c

• Support for FROM tbl AS alias and JOIN tbl AS alias (the as_name field already existed in xdb_reftbl_t but was never populated).
• xdb_stmt_find_table fixed to resolve by alias — necessary to disambiguate a self-join (same table twice).
• Once the alias is assigned, the real table name is no longer resolvable (standard SQL semantics).

4. WHERE/ORDER BY Consistency on Joined Tables

Files: src/parser/xdb_parser_dml.c, src/core/xdb_crud.c, src/parser/xdb_stmt.h

• Crash fixed: a WHERE referencing only a joined table (not the base one) left the base table's index state inconsistent → NULL dereference.
• WHERE filter on a joined table never applied: conditions were parsed but ignored at execution time (xdb_sql_join only considered ON conditions).
• ORDER BY did not support qualified references (tbl.col) — it failed even on simple JOINs.
• Row corruption bug in ORDER BY/LIMIT: both functions assumed one row = one pointer, whereas a JOIN result is a batch of reftbl_count consecutive pointers; sorting/truncation shuffled single pointers instead of whole batches, producing rows with columns mismatched across different tables. Fixed with the correct stride and a new order_reftbl_id field (constraint: all ORDER BY columns must belong to the same joined table).
• Also fixed a relative indexing bug (pRefTbl[i] instead of pStmt->ref_tbl[i]) that broke multi-table resolution after the first table switch.

5. Tests Added

Files: test/xdb_smoke_ddl.c, test/xdb_smoke_dml.c

18 new test cases (139 → 147 counting the double fixture instances), including: FK crash, FK enforcement (insert-check + RESTRICT), 2/3-table join, join with index, non-chained join rejected, combined join + WHERE + ORDER BY + LIMIT, ambiguous column, table alias, self-join. Full suite verified with no regressions under ASan.