docs: add TIME_STREAM_PLAN.md for PPS callback consumer work

[ringof]

Summary

Adds TIME_STREAM_PLAN.md at the repo root. This is the working
plan for consuming the upcoming librx888 PPS callback API
(rx888-tools branch claude/detect-dropped-samples-YbnYV,
documented in doc/pps_window.md) and exposing UTC-aligned
rx_time stream tags + a status message port on the
rx888.source block.

Locked decisions captured in the plan:

• Hard-require the new librx888 via CMake check_symbol_exists
  (no pkg-config version literal).
• Host system clock (CLOCK_REALTIME) as the UTC source for this
  first pass; gpsd lands in a follow-up PR.
• Bundle the async status message port (pps_window,
  host_overflow, device_gone, firmware_version,
  clock_locked) with the time-stream work.
• Always emit a pps_window message per PPS event (with a
  deficit_samples field), not only on anomaly.
• No new GRC parameters for time-tagging behaviour — emit when
  librx888 delivers events, silent otherwise.

This PR is plan-only. Implementation lands in follow-up PRs once
the plan is signed off.

Test plan

• Plan reviewed against rx888-tools/doc/pps_window.md on
  branch claude/detect-dropped-samples-YbnYV for API shape
  parity (struct fields, callback signature, thread rules).
• Decisions confirmed before implementation work starts on
  lib/source_impl.{h,cc}, tests/mock/librx888.{h,c}, and
  CMakeLists.txt.

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