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>

hbrtl_ext/bridge_capi/capi.go

@@ -0,0 +1,175 @@
+// Package bridge_capi re-implements the seven C helpers that
+// fivenode's PRG layer expects (originally provided by
+// fivenode_capi.c + fivenode_buffers.c). The originals back their
+// state with process-global static buffers — safe under Node's
+// single-threaded N-API caller, unsafe for fivenode_go where every
+// HTTP request runs on its own goroutine and *hbrt.Thread.
+//
+// Here, state is keyed by *hbrt.Thread so concurrent requests don't
+// stomp on each other's output buffer or context JSON.
+//
+// PRG-visible names (identical to fivenode):
+//
+//	_CTX_SET_JSON(cJson)        — store the request context as JSON
+//	_CTX_GET_JSON() -> cJson    — read it back
+//	_OUT_APPEND(cText)          — append to the response body buffer
+//	_OUT_GET()    -> cText      — read the buffer (host calls this
+//	                              after the PRG handler returns)
+//	_OUT_CLEAR()                — reset the buffer
+//	_BRIDGE_SET_RESULT(cText)   — record a fast-path response string
+//	_BRIDGE_GET_RESULT() -> cText
+//
+// Memory lifecycle: each request goroutine ends up creating a fresh
+// Thread, so the per-thread state in stateMap accumulates as the
+// process runs. The HTTP dispatcher (or any other host) calls
+// CleanupThread once the request is done — see the public helper
+// near the bottom of this file. Without that hook, state for dead
+// threads would leak; with it, the map stays bounded by the number
+// of in-flight requests.
+package bridge_capi
+
+import (
+	"bytes"
+	"sync"
+
+	"five/hbrt"
+)
+
+type state struct {
+	mu       sync.Mutex
+	ctxJSON  string
+	output   bytes.Buffer
+	resultS  string // _BRIDGE_SET_RESULT payload (separate from output buffer)
+	resultOK bool   // distinguishes "" from "never set"
+}
+
+var stateMap sync.Map // map[*hbrt.Thread]*state
+
+func stateFor(t *hbrt.Thread) *state {
+	if v, ok := stateMap.Load(t); ok {
+		return v.(*state)
+	}
+	s := &state{}
+	actual, _ := stateMap.LoadOrStore(t, s)
+	return actual.(*state)
+}
+
+// CleanupThread releases the per-thread state. The HTTP dispatcher
+// invokes this in a defer once the handler returns so the map stays
+// bounded by in-flight requests rather than total requests served.
+func CleanupThread(t *hbrt.Thread) {
+	stateMap.Delete(t)
+}
+
+// OutputBytes returns a copy of the per-thread output buffer for the
+// host to write to the wire after the handler finishes. Returning a
+// copy (rather than the underlying slice) avoids any chance of a
+// later _OUT_APPEND racing the writer.
+func OutputBytes(t *hbrt.Thread) []byte {
+	s := stateFor(t)
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.output.Len() == 0 {
+		return nil
+	}
+	out := make([]byte, s.output.Len())
+	copy(out, s.output.Bytes())
+	return out
+}
+
+// Result returns the fast-path result payload if PRG set one via
+// _BRIDGE_SET_RESULT, plus an ok flag so callers can distinguish
+// "explicitly empty" from "never set".
+func Result(t *hbrt.Thread) (string, bool) {
+	s := stateFor(t)
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.resultS, s.resultOK
+}
+
+// SetContextJSON lets the host seed the per-thread context before
+// invoking the PRG handler. It's a Go-side mirror of _CTX_SET_JSON
+// so the HTTP dispatcher can hand the request hash to PRG without
+// going back through PRG itself.
+func SetContextJSON(t *hbrt.Thread, jsonStr string) {
+	s := stateFor(t)
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.ctxJSON = jsonStr
+}
+
+func init() {
+	hbrt.HB_FUNC("_CTX_SET_JSON", capiCtxSetJSON)
+	hbrt.HB_FUNC("_CTX_GET_JSON", capiCtxGetJSON)
+	hbrt.HB_FUNC("_OUT_APPEND", capiOutAppend)
+	hbrt.HB_FUNC("_OUT_GET", capiOutGet)
+	hbrt.HB_FUNC("_OUT_CLEAR", capiOutClear)
+	hbrt.HB_FUNC("_BRIDGE_SET_RESULT", capiBridgeSetResult)
+	hbrt.HB_FUNC("_BRIDGE_GET_RESULT", capiBridgeGetResult)
+}
+
+func capiCtxSetJSON(ctx *hbrt.HBContext) {
+	if ctx.PCount() < 1 || !ctx.IsChar(1) {
+		ctx.RetNil()
+		return
+	}
+	SetContextJSON(ctx.T, ctx.ParC(1))
+	ctx.RetNil()
+}
+
+func capiCtxGetJSON(ctx *hbrt.HBContext) {
+	s := stateFor(ctx.T)
+	s.mu.Lock()
+	v := s.ctxJSON
+	s.mu.Unlock()
+	ctx.RetC(v)
+}
+
+func capiOutAppend(ctx *hbrt.HBContext) {
+	if ctx.PCount() < 1 || !ctx.IsChar(1) {
+		ctx.RetNil()
+		return
+	}
+	s := stateFor(ctx.T)
+	s.mu.Lock()
+	s.output.WriteString(ctx.ParC(1))
+	s.mu.Unlock()
+	ctx.RetNil()
+}
+
+func capiOutGet(ctx *hbrt.HBContext) {
+	s := stateFor(ctx.T)
+	s.mu.Lock()
+	v := s.output.String()
+	s.mu.Unlock()
+	ctx.RetC(v)
+}
+
+func capiOutClear(ctx *hbrt.HBContext) {
+	s := stateFor(ctx.T)
+	s.mu.Lock()
+	s.output.Reset()
+	s.mu.Unlock()
+	ctx.RetNil()
+}
+
+func capiBridgeSetResult(ctx *hbrt.HBContext) {
+	s := stateFor(ctx.T)
+	s.mu.Lock()
+	if ctx.PCount() >= 1 && ctx.IsChar(1) {
+		s.resultS = ctx.ParC(1)
+	} else {
+		s.resultS = ""
+	}
+	s.resultOK = true
+	s.mu.Unlock()
+	ctx.RetNil()
+}
+
+func capiBridgeGetResult(ctx *hbrt.HBContext) {
+	s := stateFor(ctx.T)
+	s.mu.Lock()
+	v := s.resultS
+	s.mu.Unlock()
+	ctx.RetC(v)
+}