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feat(bb_led): smooth dim breathing — 16-bit set_level + CIE gamma (#331)

bb_led_anim BREATHE/PULSE/SOLID quantized brightness to integer percent and the
backends mapped it linearly, so a dim (e.g. 0-5%) breathe rendered as ~6 visible
steps. Add a fine-resolution, gamma-corrected brightness path so dim animations
are smooth.

- bb_led: new bb_led_set_level(h, idx, uint16_t level) + an optional set_level
  vtable op; the core bridges to set_brightness for drivers without it (gpio,
  test mocks), so existing drivers and direct callers are unchanged.
- bb_led_gamma: shared CIE 1931 L* curve (257-entry LUT, ~514 B, no <math.h>);
  backends map a perceptual level to linear output.
- backends: bb_led_pwm (espidf/host/arduino) -> full resolution_bits duty;
  bb_led_apa102 (espidf/host/arduino) -> scale the base color by gamma at 8-bit
  per-channel (global=31), preserving hue, instead of the coarse 5-bit global.
- bb_led_anim: breathe/pulse/solid interpolate at 16-bit and call set_level. The
  public pattern struct is unchanged (uint8_t percent) - consumers get smooth dim
  output for free on a re-pin.

host: +7 tests (level bridge + direct call, PWM/APA102 gamma applied, 0-5% breathe
yields >=20 distinct levels); 1135/1135 pass. on-device: the TaipanMiner S2 mining
breathe renders as a smooth dim glow (at 13-bit PWM).

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

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