[QEC] Decoding server: serve HOST_CALL requests inline; retire the worker threads, queues, and CqrTransceiver

[cketcham2333]

Summary

Retires the cross-thread request path in the QEC decoding server ("fix A" of the
decoding-server redesign). Every host RPC previously crossed two server threads:
shim → CqrTransceiver::inject (frame copy, token, pending map) → RpcDispatcher
re-parse/re-route → WorkItem copy → bounded queue → per-decoder worker thread →
decoder → transport.send → pending lookup → tx-slot memcpy → promise wakeup of
the parked dispatcher — two thread crossings and ~5 buffers per get_corrections.

Now the shim resolves the DecodingSession by the payload's decoder_id and
calls its handle_* method inline on the CUDAQ dispatcher thread: the rx
slot is parsed in place, the decoder runs on the calling thread, and the
response is written straight into the tx slot (magic release-stored last).
Zero thread crossings, zero copies, zero steady-state allocations — the same
shape as the GPU device-graph dispatch path and the legacy direct path
(host::enqueue_syndromes), whose validation ladder, capture hook, and pin
helper handle_enqueue now shares.

Net: +960 / −2,702 lines.

Measured latency (udp two-process rig, d3 surface code, pymatching, 500 shots)

RPC	before (median)	after (median)	p99 after
enqueue_syndromes	0.8 µs	0.10 µs	0.86 µs
get_corrections	1.6 µs	0.08 µs	0.10 µs
reset_decoder	4.8 µs	0.10 µs	0.10 µs

(get_corrections no longer contains the decode wait — the decode runs inline
in the volume-completing enqueue, whose tail carries the actual decode time.)

What's new

• RpcSlot.h — single home for in-place slot parsing and in-place response
  writing; deliberately device-portable (no allocation/exceptions/STL) so the
  GPU dispatch kernel can later compile the same source.
• DecodingSession — payload-level cores plus inline handle_* entry
  points; corrections pack directly into the tx slot (no truncation: an
  oversized result fails with INTERNAL_ERROR).
• Debug-only single-caller guard — a scoped in-flight assert catches the
  invalid two-rings-one-decoder topology loudly in debug builds and compiles
  out entirely in release (invalid-config guards must not add hot-path
  latency). Always-on enforcement is deliberately omitted: the HOST_CALL ABI
  carries no ring identity to validate against, and multi-sender serialization
  belongs to the multi-ring decoding-unit design.
• pin_decode_device_cached — thread-local-cached CUDA pin; pin failures
  surface at server bring-up via a create-time probe.
• HopStats — slimmed to single-thread stage stats (parse/decode/respond);
  QEC_DECODING_SERVER_HOP_STATS and the total metric keep their semantics.

What's deleted (~1,500 LOC)

• CqrTransceiver.h (singleton transceiver: inject, tokens, pending map,
  promises, inbox, frame builders)
• RpcDispatcher + ResponseWriter (second-stage re-parse/re-route)
• DecodingSession worker machinery (WorkItem, bounded queue, worker thread,
  syndromes_dropped latch — backpressure is the transport ring; the
  SYNDROMES_DROPPED/BUSY codes stay reserved on the wire but are no longer
  emitted)
• RoundAccumulator + SyndromeMappingTable (unreachable fragment-merge
  machinery; the syndrome_mapping_id wire field stays — id 0 accepted,
  others rejected with BAD_REQUEST)
• SpinPolicy.h + QEC_DECODING_SERVER_SPIN_US (the cross-thread waits they
  optimized no longer exist)
• LoopbackTransceiver, the recv loop, install_dispatch_sink

Behavior notes

• Enqueue response moves to the handler tail; client-visible timing is
  unchanged (the tx flag publishes on handler return). The deferred-error
  contract is unchanged.
• A volume-completing enqueue runs its decode inline and blocks only its own
  ring: decoder parallelism comes from per-decoder rings (one dispatcher
  thread each). A shared ring still routes multiple decoders correctly but
  serializes their decodes.
• Test-harness updates accordingly: surface_code-4-yaml-test.sh exports the
  per-decoder ring ports (all tokens required, fail-fast), and
  hard-patch-pymatching runs 500 shots so its opportunistic overlap evidence
  is statistically certain (the deterministic proof is the three-way-overlap
  barrier test).
• run_realtime_decoding.sh: fixes a hololink→hsb rename leftover
  (--hololink → --hsb-ip) needed for --source fpga runs.

[vedika-saravanan]

Thanks Chuck Overall, this is a strong simplification of the host dispatch path. I left one small question about documenting the current one-ring-per-decoder ownership assumption as multi-ring support evolves.

[melody-ren]

Thanks Chuck! Looks good to me. Just some minor nits