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fix(bb_display): wait for SPI DMA completion before reusing bounce buffer (per-glyph draw_text corruption on ILI9341/st77xx) (#709)

- esp_lcd_panel_draw_bitmap() is async (queues the DMA transfer, trans_queue_depth=10) but bb_display_blit_spi/bb_display_clear_spi refill the shared s_bounce buffer on the next call before the prior transfer drains
- bb_display_draw_text() blits one glyph at a time, so larger glyphs (8x16, 256B) take longer to drain than smaller ones (5x8, 40B), garbling multi-glyph text on ILI9341/st77xx panels
- serialize the blit by registering an on_color_trans_done callback at panel-IO creation (bb_display_spi_new_panel_io) that signals a binary semaphore, taken after every draw_bitmap chunk before the buffer is reused
- affects any SPI-panel draw_text consumer sharing bb_display_spi_common (ili9341 + st77xx); assumes the existing single-drawing-task contract — the semaphore gates DMA completion, not cross-task sync

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

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