Merge tag '8.0.0-alpha.0' into claude-merge-1

[dimoffon]

The beginning of ggdb 8 development. Just enough to pass the basic test set.

Fix all compilation errors introduced by the PG13 API changes:

List API changes (PG13):

• lnext(cell) -> lnext(list, cell) throughout src/, contrib/, gpcontrib/, and unit test files (lsyscache_test.c, fixedwidth.c, extaccess.c, etc.)
• for_each_cell(cell, initcell) -> for_each_cell(cell, list, initcell)
• list_delete_cell(list, cell, prev) -> list_delete_cell(list, cell) or foreach_delete_current(list, cell) inside foreach loops; remove prev tracking
• list_qsort(list, cmp) -> list_sort(list, cmp); list_sort returns void, so drop any assignment of its return value
• lappend_cell(list, prev, datum) removed; replace with lappend(list, datum)
• ListCell is now a flat union: cell->data.ptr_value -> cell->ptr_value

ORCA C++ wrappers (src/include/gpopt/gpdbwrappers.h):

• Remove 'struct ListCell;' forward declaration (ListCell is now a union)
• Update ForEach, ForBoth, ForThree, ForEachWithCount macros to 2-arg lnext

Storage manager vtable (src/backend/storage/smgr/smgr.c):

• smgr_open removed from f_smgr vtable; call mdopen(reln) directly

Base64 API (PG13 added dstlen parameter):

• pg_b64_encode(src, len, dst) -> pg_b64_encode(src, len, dst, dstlen)
• pg_b64_decode(src, len, dst) -> pg_b64_decode(src, len, dst, dstlen)

Catalog OID conflicts:

• AO_ROW_TABLE_AM_HANDLER_OID reassigned 4187->4142 (upstream took 4187)
• AO_COLUMN_TABLE_AM_HANDLER_OID reassigned 4188->4191 (upstream took 4188)
• Bump CATALOG_VERSION_NO to 302605201

Header cleanup:

• src/bin/scripts/common.h: remove duplicate appendQualifiedRelation declaration left by conflict resolution (4-arg vs 5-arg forms)

Build script:

• concourse/scripts/compile_gpdb.bash: guard 'git describe --tags' with '|| true' and skip changelog generation when no tags are reachable

Merge rules documentation:

• GG_PG_MERGE_RULES.md: add all newly discovered patterns to the error table and verification checklist

Here are some reminders before you submit the pull request

• Add tests for the change
• Document changes
• Communicate in the mailing list if needed
• Pass make installcheck
• Review a PR in return to support the community