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fix(display): collapse function-static row buffers into one shared union (TA-302) (#312)

components/display/src/display.c had four function-static scratch arrays
on the ST7735 path that all lived in BSS for the life of the process:

  clear_st7735::s_line[LCD_WIDTH]              320 B
  draw_text_st7735::s_char_buf[2][FONT_W*FONT_H]  512 B
  draw_logo_st7735::s_logo_row[2][LOGO_W]      224 B
  render_frame::s_row_buf[2][LCD_WIDTH]        640 B
  ──────────────────────────────────────────  ──────
                                               1696 B

None of these execute concurrently — the display task is single-threaded
and each render function fully completes before the next runs. The next
esp_lcd_panel_draw_bitmap drains any in-flight DMA from the prior call
before reusing the buffer (already documented in the existing
double-buffer comment).

Replace all four with one shared union sized to the largest member
(render_frame's row pair, 640 B). Net BSS saving: ~1056 B on tdongle-s3
builds. Bitaxe builds (SSD1306, no ST7735 path) unaffected.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

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