docs: prune TESTING.md to its forward-looking content

The 730-line milestone-by-milestone bring-up log served its purpose —
every milestone (M1-M9) is verified on hardware, and the per-run
narratives are preserved in git history (last full version: 8eebc0e).
What survives, restructured (~230 lines):

- New-unit bring-up: hardware prereqs (all unknowns resolved), the
  flash -> panel -> touch -> Scrypted sequence, and the panel
  failure-mode signals table (the content you actually need the day a
  second viewport gets provisioned).
- Regression recipes: condensed curl checks for config validation (incl.
  the 54-char name cap), frame push, state machine, OTA acceptance, and
  device-initiated POSTs.
- Outstanding tests: the honestly-never-run backlog (mid-frame cable
  pull, 24h soak, frame storm, brown-out, multi-viewport).
- Performance playbook: kept, with example log formats refreshed to the
  current stream instrumentation (the old examples showed the removed
  pre-stream snapshot path) and thresholds updated to the post-tuning
  baseline (24fps painted, pend_age as the backlog tripwire).

README: pointers at TESTING's old structure updated; the Hardware status
row no longer claims schematic confirmation is pending (those unknowns
were resolved during bring-up — only the per-unit flash-size check
remains).

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