feat(forward): rate cap + seed-state mode for safe backfill (v1.5.2)

Two safety nets for first-deploy on Blastware ACH machines that
have accumulated tens or hundreds of thousands of historical events
in the watch folder.

1. SFM_MAX_FORWARDS_PER_PASS (default 500, 0=unlimited)
   ---------------------------------------------------
   Cap on the number of events forwarded per scan tick.  At the
   60-second default interval that's ~30K events/hour throughput —
   the SFM server gets a steady drip instead of one giant burst.
   Scan now sorts by mtime ascending so backfill advances
   chronologically (oldest first) and successive scans always
   make progress instead of re-considering the same N newest files.

   Wired into:
     - event_forwarder.find_pending_events / forward_pending
     - series3_watcher.run_watcher loop
     - config-template.ini
     - settings_dialog SFM Forward tab (new "Max Events Per Pass"
       spinbox, validated in _on_save)

2. event_forwarder.py --seed-state CLI
   -----------------------------------
   One-shot mode that walks the watch folder, sha256s every in-window
   event binary, and marks them all as already-forwarded WITHOUT
   POSTing anything.  Run before flipping SFM_FORWARD_ENABLED=true
   to skip the historical backfill entirely — the watcher then only
   forwards events that appear AFTER the seed.

   Usage:
       python event_forwarder.py --seed-state \
           --watch "C:\Blastware 10\Event\autocall home" \
           --state "C:\...\sfm_forwarded.json" \
           [--max-age-days 365]

7 new unit tests:
  - max_per_pass cap enforcement (=N, =0 unlimited, oldest-first
    ordering)
  - seed-state mode (in-window seeding, max-age skip,
    end-to-end skip-after-seed, idempotent re-runs)

README adds a "First-time deployment" section walking through both
options.  Bumps to v1.5.2.

series3_tray.py

@@ -1,5 +1,5 @@
 """
-Series 3 Watcher — System Tray Launcher  v1.5.1
+Series 3 Watcher — System Tray Launcher  v1.5.2
 Requires: pystray, Pillow, tkinter (stdlib)
 
 Run with:  pythonw series3_tray.py   (no console window)