testfixture: page-image APIs unsupported (sqlite3_backup_*, sqlite3_serialize/deserialize, memdb VFS)

[timsehn]

test/known_testfixture_exception_inventory.txt marks 6 inherited SQLite
testfixture suites unsupported because they exercise APIs that hand out or
consume a raw SQLite page image. DoltLite stores content-addressed prolly
chunks and has no stock page image to produce, so:

• sqlite3_backup_init rejects the page-image backup (backup, backup2,
  backup4, backup5, backup_malloc)
• sqlite3_serialize returns NULL cleanly and the following
  sqlite3_deserialize aborts the file (memdb1)
• the chunk store deliberately refuses the memdb VFS at open, so db is never
  created (memdb2)

DoltLite exports through a VFS-level backup (sqlite3_js_db_export under
DOLTLITE_PROLLY) rather than the page-image API. Not expected to be fixed.

The issue exists so the inventory's unsupported entries point at a live
description of the limitation. backup* and memdb2 previously pointed at
#1208 (the closed triage process tracker) and memdb1 at #1225 — which was
the narrower "ASan: sqlite3_serialize NULL deref" bug and was genuinely fixed,
so it no longer describes why the suite is still excepted.

Affected suites (7): backup backup2 backup4 backup5 backup_malloc memdb1 memdb2

[timsehn]

Update: file-backed and attached DoltLite backup support landed in #2255, and #2258 adds native file-to-memory, memory-to-file, and memory-to-memory backup. The remaining serialize/deserialize work is tracked separately in #2257; memdb VFS support also remains outside #2258.