fix(engine): don't truncate a compound index expression to a field name

[russimicro]

Table::field_index retries a lookup longer than 10 characters against name.substr(0, 10), because DBF CREATE truncates field names to 10 (a proc __output column databasepath lands as DATABASEPA).

An index expression whose first component is a 10-character field — CCODIGOCON+CDOCUMETRA, CPREFIJTRA+CDOCUMETRA — therefore matched that field and reported itself as a bare field name. AdsCreateIndex61 then took the bare-field branch and pinned the key length to the first component's width, so every later component fell out of the key: records sharing the first component collapsed onto one key and ordered by recno. GotoTop on the compound tag landed on the wrong record, and an exact AdsSeek matched the wrong row.

The retry now only fires for a plain identifier (alphanumeric/underscore), which is the case it was written for.

Test: abi_cdx_estaelec_compound_test — two distinct compound tags over the same table (con+doc and pre+doc) must order differently, plus a computed DTOS() tag, a FOR-clause tag, and an exact seek on a compound key.

Found while running a Harbour/FiveWin ERP's real index set on OpenADS.

Suite: 1179/1191 — the same 12 pre-existing SQL-parser (7200) failures as pristine v1.8.31, verified on this branch in isolation (MSVC x64 Release).

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