[BUG] Pervasive `SELECT true ... LIMIT 1` in `existed` checks is non-portable to Oracle

- severity: Medium
- files: `selector-sqlmap.xml:209-214`; `rule-sqlmap.xml:156-161`; `app-auth-sqlmap.xml:140-145,147-152`; `plugin-sqlmap.xml:136-140,142-146`; `role-sqlmap.xml:79-84`; `auth-path-sqlmap.xml:84-96`; `data-permission-sqlmap.xml:112-117`; `meta-data-sqlmap.xml:165-170`; `namespace-plugin-rel-sqlmap.xml:43-47`; `tag-sqlmap.xml:41-46`; etc.
- description: Dozens of `existed`/`*Existed` probes use `SELECT true FROM t WHERE ... LIMIT 1` returning `java.lang.Boolean`. `LIMIT 1` is invalid on Oracle (needs `ROWNUM`/`FETCH FIRST`) and `SELECT true` is not valid Oracle SQL (Oracle has no boolean type). Only `plugin` and `operation-record-log` carry `databaseId="oracle"` variants; none of these `existed` statements do.
- impact: Any `existed`/uniqueness probe throws on Oracle; the few Oracle-specific overrides show Oracle is a supported target.
- suggested_fix: Standardize on `SELECT 1 FROM ... WHERE ... LIMIT 1` (or `FETCH FIRST 1 ROWS ONLY`) and add `databaseId="oracle"` variants.
- confidence: Medium
- related_existing: partially overlaps #6161 (Oracle schema DDL) and #5631 (Oracle rule-sqlmap `;`), but this is a distinct, broad mapper-SQL portability defect across the `existed` family.

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/06-medium-tiers.md`](docs/scan2-2026-08-02/06-medium-tiers.md)._

Start with the listed mapper files, including selector-sqlmap.xml, rule-sqlmap.xml, app-auth-sqlmap.xml, plugin-sqlmap.xml, and role-sqlmap.xml, then compare the existing databaseId="oracle" variants in plugin and operation-record-log. Trace every existed/*Existed statement in the scan list and verify that the completed mapper SQL works for both the default database and Oracle.

Written by the indexing model from the issue text.