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feat(bb_display): st77xx backend (ST7735 + ST7789) + probe scaffold (B1-133) (#181)

extends the bb_display_backend_t vtable with an optional probe()
slot (NULL = always-succeed) and converts the core registry from
a single backend pointer to an ordered list. bb_display_init now
walks registrations in insertion order, picking the first whose
probe()->init() succeeds. existing single-backend builds (ek79007)
get probe=NULL and behave identically to today.

new bb_display_st77xx component ships two backends, kconfig-
selected:
- ST7735 — overlays a vendor init sequence (gamma/power/frame)
  on top of esp_lcd's st7789 driver, needed for the lilygo
  t-dongle s3 panel where the stock driver doesn't program the
  chip correctly. opt-out via CONFIG_BB_DISPLAY_ST7735_SKIP_VENDOR_INIT
  for panels that don't need the overlay.
- ST7789 — generic stock-driver init, no overlay.

both register with probe=NULL today because the only consumer
hardware (tdongle) doesn't wire SPI MISO and can't read RDDID.
real probe() lands in a follow-up when probe-capable hardware
arrives (B1-138 tracks the schematic check).

tdongle smoke env wires bb_display + bb_display_st77xx with the
ST7735 variant. all four smoke envs build green; ek79007 stays
isolated to the elecrow target.

pr3 (B1-134, ssd1306) follows; tm display retirement waits for
pr3 + tag.

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