feat(lamni): min-projection smear composite for absorption contrast

The smear rotation-center aid built a max-projection composite, which suits
bright features on a dark background. On the X-ray eye the sample shows up
in absorption (dark on bright), and a max-projection washes those features
out because every pixel sees the bright background at some point during the
rotation.

Add projection="min"|"max" to _smear_sweep(),
find_rotation_center_smear_experimental() and
lamni.xrayeye_rotation_center_calibration_smear_experimental(), defaulting
to "min". Invalid values raise before any motion; the choice is stored in
the calibration HDF5. User and developer docs updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/developer/lamni_smear_architecture.md

@@ -123,9 +123,19 @@ stepping the motor:
    finish.
 2. A plain loop polls `report.status` (exactly what `ScanReport.wait()` does
    internally) and, while it isn't `"COMPLETED"`, grabs a frame via
-   `get_last_image()` and folds it into a running `np.maximum` composite —
+   `get_last_image()` and folds it into a running composite —
    all single-threaded, since the rotation is progressing concurrently on
-   the server regardless of what the client does between polls.
+   the server regardless of what the client does between polls. The
+   composite is `np.minimum` for `projection="min"` (default; absorption
+   contrast, dark features on a bright background) or `np.maximum` for
+   `projection="max"` (bright features on a dark background). A
+   max-projection is useless for absorption: the bright background passes
+   every pixel at some point during the rotation and washes the dark
+   features out. Taking the minimum is equivalent to inverting the frames,
+   taking the maximum and inverting back, without needing the camera's
+   full-scale value. The value is validated up front (`ValueError` before
+   any motion) and stored as the `projection` attribute in the calibration
+   HDF5.
 3. Every `display_update_interval_s`, the current composite is pushed to
    `smear_preview` so the operator watches it grow; off-axis features trace
    arcs as the sample turns, and the common center of curvature of those