Revert the detection floor: it guards the wrong resolution

Reverts 53f7cdf and e92d22d. The floor those added sat on
Detector::detect, refusing any buffer under 1025px on the reasoning that
a small buffer finds no faces. That reasoning does not survive §4.1:
the detector letterboxes every input to 640x640, so a face occupying 2%
of the frame presents at 12px to the model whether it is handed a 1024px
buffer or a 6000px one. Detector input is precisely the quantity that
does not matter.

Worse than merely useless, it blocks the design FR-CULL-8 now specifies,
where the detector is deliberately fed a downscale and the crop is taken
from the native render. A guard on detect() rejects exactly that call.

What the measurement actually supports is a floor on the *crop* source,
which is where resolution converts into embedding quality, and which
faces.crop_px already records: 47% of the reference library's faces were
upsampled to reach 112x112. That floor is a separate change against the
native-resolution path and does not belong on the detector.

The 23x faces-per-image gap by source_edge that motivated the original
commit is kept in faces.md §7b, restated as the unexplained observation
it is rather than the causal claim it was written as. V12 stands: those
runs cropped at 1024 whatever detection did, and that is reason enough
to look at them again.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-face/src/lib.rs

@@ -43,31 +43,6 @@ pub mod embedding;
 pub mod naming;
 pub mod neighbours;
 
-/// Smallest proxy long edge detection will accept, in pixels.
-///
-/// # Why this exists, and why it is 1025 rather than 1024
-///
-/// A proxy is letterboxed into the detector's 640px input whatever size it
-/// arrives at, so a small one does not fail — it silently finds almost
-/// nothing, and the
-/// caller records a run marker saying the image was examined. That marker is
-/// the damage: it is indistinguishable from "examined properly, no faces
-/// here", so the photograph is never looked at again.
-///
-/// The number is not a guess. Measured over a 23,500 image library, detection
-/// against a 1024-edge proxy returned **0.078 faces per image**; the same
-/// files — DNGs, 21 MB against 23 MB, so the same photographs in every respect
-/// that matters — returned **1.82** against a proxy of 2048 or better. A
-/// twenty-three fold difference, on identical source material and identical
-/// weights. Ninety percent of the 1024-edge runs found nothing at all.
-///
-/// 1024 is exactly `dr_thumbs::ThumbSize::Large`, the tier a proxy is stored
-/// at, and it was the tier one of the two sweeps detected on. So the floor has
-/// to *exclude* 1024 rather than admit it, and the constant is one past it:
-/// `edge < MIN_DETECT_EDGE` is then the whole test, with no off-by-one left at
-/// the call sites to get wrong.
-pub const MIN_DETECT_EDGE: u32 = 1025;
-
 pub use align::{warp, Aligned112, Similarity, ALIGNED_EDGE, ARCFACE_TEMPLATE};
 pub use assign::{identity_shares, RIVAL_FLOOR, TOP_MATCHES};
 pub use calibrate::{Calibration, Pairs, ReliabilityBand};
@@ -105,14 +80,6 @@ pub enum FaceError {
 
     #[error("image buffer is {got} floats, expected {expected} (RGB, three per pixel)")]
     ImageShape { expected: usize, got: usize },
-
-    /// The proxy is too small to detect on, so nothing was attempted.
-    ///
-    /// A distinct variant because the caller must treat it differently from a
-    /// failure: nothing is wrong with the image or the model, and the right
-    /// response is to find better pixels rather than to retry these.
-    #[error("proxy long edge is {edge}px, below the {needed}px detection floor")]
-    ProxyTooSmall { edge: u32, needed: u32 },
 }
 
 /// Install tract as `ort`'s backend.