fix(CLI-40,CLI-41,CLI-44): exact-secret scrub, uniform malformed-reply contract, Go terminal-error mislabel

CLI-40: port the exact-secret credential scrub to Rust/Java/.NET (Go/Python
already did it). AuthenticateUser/WriteSecured(2) helpers now redact the exact
caller-supplied secret from any surfaced error, as defense-in-depth on top of the
by-construction guarantee. Rust hand-writes a redacting Debug (derived Debug would
leak the reply); Java/.NET rebuild the same exception type with the redacted
message and do not carry the secret-bearing original forward (so ToString/stack
traces stay clean too).

CLI-41: uniform malformed-reply contract for AuthenticateUser/ArchestrAUserToId/
AddBufferedItem across all five clients — typed payload, else a present int32
return_value, else a typed malformed-reply error. Fixes Go/Java silent-0, .NET
NRE, and Rust's own internal inconsistency.

CLI-44: the Go event goroutine's Recv-error path now uses a non-blocking
sendTerminalEventResult on the reserved slot, so a genuine terminal stream error
is reported as itself instead of being mislabeled ErrSlowConsumer under overflow.

Riders from the CLI-37/38 review: (a) .NET ToDiagnosticSummary and Python
_mxaccess_message surface the raw success member (diagnostics-only parity with
Rust); (b) the status-conversion fixture carries an independent wantSuccess
boolean and the Go/.NET fixture tests assert against it instead of recomputing
the formula under test.

Shared fixtures (authenticate-user.{echoed-credential,missing-payload,
return-value-only}.reply.json) + manifest + ClientBehaviorFixtures.md +
ClientLibrariesDesign.md updated in the same change. Tracking: CLI-40/41/44 -> Done.

clients/proto/fixtures/behavior/statuses/status-conversion-cases.json

@@ -3,6 +3,7 @@
   "cases": [
     {
       "id": "ok.responding-lmx",
+      "wantSuccess": true,
       "status": {
         "success": 1,
         "category": "MX_STATUS_CATEGORY_OK",
@@ -15,6 +16,7 @@
     },
     {
       "id": "security-error.requesting-lmx",
+      "wantSuccess": false,
       "status": {
         "success": 0,
         "category": "MX_STATUS_CATEGORY_SECURITY_ERROR",
@@ -27,6 +29,7 @@
     },
     {
       "id": "raw-unknown-category",
+      "wantSuccess": false,
       "status": {
         "success": 0,
         "category": "MX_STATUS_CATEGORY_UNKNOWN",