SQL backends: wire write (dbAppend/REPLACE/dbDelete) into the navigational ABI + emulate rLock()/fLock() (SQLRDD parity)

[Admnwk]

Summary

The SQL backends (SQLite, PostgreSQL, MariaDB/MySQL, MSSQL, Firebird, ODBC) expose read + navigation + SEEK through the ACE ABI, so a Harbour app using rddads works with the standard xBase navigational commands (dbGoTop/dbSkip/dbSeek, FieldGet) — much like SQLRDD. However, the write surface is not yet wired into the navigational ABI for any SQL backend, and record/file locking (rLock()/fLock()) is not emulated on any SQL backend.

This issue tracks closing that gap so the SQL backends become full read/write through the xBase API (SQLRDD-equivalent).

Current state (verified)

Capability	SQLite	PostgreSQL	MariaDB	MSSQL	Firebird	ODBC
Read + navigation	✅	✅	✅	✅	✅	✅
dbSeek (by column/index)	✅	✅	✅	✅	✅	✅
Write (dbAppend/REPLACE/dbDelete) via ABI	❌	❌	❌	❌	❌*	❌*
rLock() / fLock()	❌	❌	❌	❌	❌	❌

Details:

• AdsAppendRecord / AdsSetString / AdsDeleteRecord (in src/abi/ace_exports.cpp) only route through the remote/wire connection (tcp://) or the native local DBF table. There is no get_firebird_table / get_odbc_table / get_maria_table / get_postgres_table branch in any write function (read functions do use those getters, 18–20× each).
• (*) FirebirdConnection::append_blank/set_field/delete_record (staged INSERT/UPDATE/DELETE) and the ODBC connection write methods exist and are unit-tested at the connection layer (tests/unit/firebird_connection_test.cpp), but are only called by those tests — never by the ABI.
• MariaDB / PostgreSQL / MSSQL connections have no write methods (AdsAppendRecord on MSSQL explicitly returns "write not available in v1"). docs/OPENADS_PLUS.md marks PostgreSQL Write as "Planned".
• No SQL backend defines lock_record / lock_table.

Proposed plan (cheap → expensive)

1. Wire Firebird write into the navigational ABI — AdsAppendRecord, AdsSetString/SetDouble/SetLong/... (REPLACE), AdsWriteRecord (commit row), AdsDeleteRecord, mirroring how Firebird read is already wired (get_firebird_table). The engine already exists + is unit-tested, so this is the lowest-risk first win. ABI-level test (live-FB gated, skips without OPENADS_TEST_FB_URI).
2. Wire ODBC write the same way — strategic, since ODBC is a universal gateway (write to MariaDB/PostgreSQL/etc. via odbc://DSN even before native write lands).
3. Native write for MariaDB / PostgreSQL / MSSQL — implement append_blank/set_field/delete_record on each connection (staged INSERT/UPDATE/DELETE keyed by PK), following the Firebird pattern, then wire into the ABI.
4. Emulate rLock() / fLock() — map to the SQL engine's locking/transaction semantics (e.g. SELECT ... FOR UPDATE / row versioning), the main remaining SQLRDD-parity gap.

Notes

• Changes to src/abi/ace_exports.cpp are coordinated upstream (Antonio) — delivered as additive, source-clean PRs from the fork, not merged directly.
• Backend-side write methods (src/sql_backend/*) are additive and self-contained.