Complete HMDS no-data replies without silently retrying a day
older. Let `open_history_client()` classify the original request
and leave frame stepping to `start_backfill()`.
Distinguish `request_end_dt` from `oldest_shm_dt` so the first
frame after an empty interval retains its unwritten end bar.
Inline the storage time cutoff and drop the unused two-direction
`diff_history()` helper. Explain both dates and filtering rules.
Add request-boundary tests and real SHM/NativeDB regressions:
all 2,004 supplied bars survive, while the closure stays empty.
Prompt-IO: ai/prompt-io/codex/20260923T164656Z_efa72980_prompt_io.md
(this patch was generated in some part by `codex` using
`gpt-6` (`openai`))
Gud Boi