feat(bridge_capi): port fivenode_capi.c / fivenode_buffers.c to Go RTL

Seven HB_FUNCs that fivenode's bridge_*.prg layer relies on:
  _CTX_SET_JSON / _CTX_GET_JSON  — per-request context payload
  _OUT_APPEND   / _OUT_GET / _OUT_CLEAR — response body buffer
  _BRIDGE_SET_RESULT / _BRIDGE_GET_RESULT — fast-path response

Crucially per-thread, not process-global like the original C
implementation. fivenode runs single-threaded under N-API so a static
buffer per process was fine; fivenode_go runs one *hbrt.Thread per
HTTP request goroutine, so the state is keyed by *hbrt.Thread in a
sync.Map. The HTTP dispatcher will call CleanupThread once per
request to keep the map bounded (sub-phase 1a.3-3).

Also exposes Go-side helpers (OutputBytes, Result, SetContextJSON,
CleanupThread) so the dispatcher can seed the context and harvest
the response without bouncing back through PRG.

Verified with app/capi_test.prg: all seven functions behave as
expected; combined with the Five hb_jsonDecode byref fix, ctx_get()
now correctly returns hash values rather than the fallback default.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/capi_test.prg

@@ -0,0 +1,44 @@
+// app/capi_test.prg — verifies bridge_capi RTL in isolation
+// (single thread, no HTTP). Multi-thread isolation will be exercised
+// once the dispatcher is wired in 1a.3-3.
+FUNCTION Main()
+   LOCAL cJson, cOut, cResult
+
+   ? "=== _CTX_SET_JSON / _CTX_GET_JSON ==="
+   _CTX_SET_JSON( '{"user":"alice","role":"admin"}' )
+   cJson := _CTX_GET_JSON()
+   ? "ctx json:", cJson
+   ? "ctx_get user:", ctx_get( "user", "?" )
+   ? "ctx_get role:", ctx_get( "role", "?" )
+   ? "ctx_get missing:", ctx_get( "nope", "DEFAULT" )
+
+   ? ""
+   ? "=== _OUT_APPEND / _OUT_GET / _OUT_CLEAR ==="
+   _OUT_APPEND( "Hello, " )
+   _OUT_APPEND( "world! " )
+   _OUT_APPEND( "Three." )
+   cOut := _OUT_GET()
+   ? "out before clear: [" + cOut + "]"
+   _OUT_CLEAR()
+   ? "out after  clear: [" + _OUT_GET() + "]"
+
+   ? ""
+   ? "=== _BRIDGE_SET_RESULT / _BRIDGE_GET_RESULT ==="
+   _BRIDGE_SET_RESULT( "fast-path payload" )
+   cResult := _BRIDGE_GET_RESULT()
+   ? "result:", cResult
+
+RETURN NIL
+
+// Need ctx_get here because bridge_context.prg isn't linked in yet.
+FUNCTION ctx_get( cKey, xDefault )
+   LOCAL cJson := _CTX_GET_JSON()
+   LOCAL hCtx
+   IF Empty( cJson )
+      RETURN xDefault
+   ENDIF
+   hb_jsonDecode( cJson, @hCtx )
+   IF ValType( hCtx ) == "H" .AND. hb_HHasKey( hCtx, cKey )
+      RETURN hCtx[ cKey ]
+   ENDIF
+RETURN xDefault