wboayue / stable-flight / 38476884695

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Lateral velocity I with tilt-cap anti-windup (#357) (#368)

Copter waypoint hold now trims a steady wind with a lateral velocity I,
instead of settling downwind. The integral doesn't grow along a
tilt-capped acceleration, so it can't wind up against the tilt limit.

## Changes

- `LateralControllerConfig::default()`: velocity `i` 0.4 (was 0) and
`max_integral` 12.5 m (was 100). That caps the I term at ±5 m/s², which
equals g·tan 27°, the default tilt cap.
- **Breaking:** `LateralController::control` takes `limit_mps2`: the
acceleration the caller last capped. A step whose I term pushes along it
can't lengthen the integral. It can still shrink or turn. This mirrors
ArduPilot `libraries/AC_PID/AC_PID_2D.cpp:127-146`, described and
implemented in our own words.
- `CopterGuidance` keeps the lateral loop's last raw acceleration
request. While `tilt_capped` is set, it passes that request as
`limit_mps2` on the next run. This includes a tilt cap lowered by
`set_config`.
- Docs: the `architecture.md` integrator policy, the controllers README,
and the core CHANGELOG.

## Gain choice

- ArduPilot Copter's horizontal velocity loop uses I = P/2 (P 2.0, I
1.0, IMAX 10 m/s²;
`libraries/AC_AttitudeControl/AC_PosControl.cpp:44,74-78`).
- Our P is 1.26 and `position_p` is 0.5. For a 1 m step on an ideal
double integrator:

  | I | Overshoot | Settle to 5% | Slow pole |
  |---|---|---|---|
  | 0 (before) | 2% | 4.2 s | none |
  | 0.4 | 1% | 3.5 s | 0.26 /s |
  | 0.63 (ArduPilot's I/P ratio) | slow creep | 7.9 s | — |

## Test bands

- **Changed: `holds_a_steady_offset_in_steady_wind` →
`holds_the_waypoint_in_steady_wind`.** `quad_x` in a 5 m/s wind, band
0.3 m (from #357). Measured worst offset: 0.03 m. On main it's 2.38 m,
so the test fails without this change.
- **Widened, because of the I's slow tail.** After a long leg or a gust,
the integral holds leftover error that decays at about 0.26 /s. The new
bands are the measured values plus margin:

  | Test | Old band |... (continued)

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