prepacked_cache: fix byte-accounting for evicted float matrices

[EylonKrause]

Problem

PrepackedCache tracks total cached bytes in buffers_bytes_ to enforce max_buffers_bytes_. Insertion and eviction account for the sums buffer asymmetrically for floating-point matrices, so the running total drifts.

Root cause

AllocateBuffers (prepacked_cache.cc:29-39) allocates and counts a sums buffer only for non-floating-point matrices — a float matrix adds just DataBytes to buffers_bytes_ (Get, line 100). But EjectOne (line 124) unconditionally subtracted DataBytes + SumsBytes.

A float matrix's sums_type still reports a nonzero element size: it is set to Type::Create<SumsType>() with SumsType = Scalar for floating-point scalars (create_trmul_params.h:62-69), i.e. float (size 4). So SumsBytes = layout.cols * sums_type.size > 0 (mat.h:436-439) even though no sums buffer was allocated. Each float eviction therefore over-subtracts cols * 4, letting buffers_bytes_ drift below true usage (possibly negative) so the cache silently overshoots its cap.

Fix

Subtract SumsBytes only for non-floating-point matrices, mirroring AllocateBuffers.

Verification

Pure accounting logic; reproduced with a standalone model of insert/evict (float matrix, data=1000, cols*4=256):

inserted (added to buffers_bytes_): 1000
after evict  buggy buffers_bytes_ = -256  (should be 0)  -> DRIFT/WRONG
after evict  fixed buffers_bytes_ = 0     (should be 0)  -> OK

The existing prepacked_cache_test.cc does not catch this: its dummy float PEMat leaves sums_type at the default (size 0), and no test evicts a float matrix. Happy to add a regression test that evicts a float matrix.