fix(pp): add_tile_matrix shape error when input AnnData has 0 vars

[Starlitnightly]

Problem

epione.pp.add_tile_matrix(adata, bin_size=500) crashes (reported as omicverse #840):

building tile matrix: 568 cells × 6,176,584 tiles (500 bp bins, strategy=paired-insertion)
ValueError: Data matrix has wrong shape (568, 6176584), need to be (568, 0).

The AnnData produced by import_fragments has 0 vars. add_tile_matrix assigned the (n_cells, n_tiles) tile matrix to adata.X before replacing var, so AnnData validated the new X against the current (empty) var and rejected it. Assigning var first would fail symmetrically — X and var must become consistent together.

Fix

Rebuild the object in place via AnnData._init_as_actual(...) with the new X + var (preserving obs/uns/obsm/obsp), keeping the original lazy .X assignment as a fallback for backed / AnnDataOOM inputs.

Verification

Minimal repro (0-var AnnData + a tiny fragments.tsv) now succeeds:

building tile matrix: 3 cells × 6 tiles (500 bp bins, strategy=paired-insertion)
tile matrix nnz=12
→ shape (3, 6), var = ['chr1:0-499', 'chr1:500-999', ...], mutated in place, uns preserved

Fixes omicverse/omicverse#840

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