Aggregate health backend and server stats C API

[bedipritpal]

Stage6: aggregate health backend, based on upstream793251aa afterPR190.

• Add OAdsGetServerStats for an AdsMgConnect management handle and the additive HealthJson read request. Existing management handshake/credential/opcode/resource guards remain; legacy credential-free loopback management stays read-only.
• Aggregate-only schema1 JSON: logical workarea/table-handle event-retained peaks, existing user/connection/index/lock telemetry, resolved session cap/source, wide cumulative counters and optional Linux processFD measurements. No usernames, addresses, SQL, credentials or path strings.
• NULL/short buffers report required size including NUL and write no partial JSON. Older servers rejecting the additive request return AE_FUNCTION_NOT_AVAILABLE. Replies have size/object-envelope checks, not full JSON syntax validation.
• Explicit max_sessions=0 now correctly overrides the environment/default as unlimited; default500 unchanged. INI/CLI only override when supplied. Shared-library/x86 stdcall export and backend documentation included.

20files (+536/-7). No Harbour/RDDADS or portal consumers, HTTP endpoint/listener, lock/count-cache/OPENADS_FRESH_COUNTS change, daemon-hardening exemption, CI/workflow or release change.

Caveats: management remains existing plainTCP (including credentials), not TLS merely because data transport is TLS-enabled. Snapshots are best-effort and may include the management session. Workareas include client-parked server handles, not Harbour Select() areas/OSFDs. Unknown rejection/worker-peak/parked-client metrics are null; other maxima retain existing process-global telemetry semantics and resource counts retain32-bit widths.

Credit: scoped derivation from Pritpal Bedi's released mtfix41 source6dffce5805fe9bc3822d630f46ed02b26286206e, adapted to upstream safeguards with added old-server/malformed-reply/config-source tests. No full-source identity claim.

Validation: isolated Linux normal/TLS unit+ACE+serverd builds complete; combined stage4+5+6 selection182cases/10,115assertions each. StrictClang14 warning checks pass changed Linux translation units/tests. Realdaemon JSON probes verify default/env/INI/CLI cap and source precedence, including explicitzero. Independent env probes cover invalid/negative/overflow fallback. Export verified in normal/TLS sharedlibs. Windows/macOS source-reviewed but not locally compiled/run; fullCTest/hostedCI not claimed