Support .obsm keys in render_labels(color=...)

[timtreis]

Feature request

render_labels(color=...) currently resolves keys from .obs columns and .var_names only. It would be useful to also support .obsm keys (or columns within obsm DataFrames).

Use case

In squidpy's tiling QC (sq.experimental.tl.calculate_tiling_qc), we compute several per-cell scores that logically belong in .obsm rather than polluting .obs:

# Current workaround: scores stored in .obs because sdata-plot can't read .obsm
# Ideally these would live in adata.obsm["tiling_qc"]
for col in combined.columns:
    adata.obs[col] = combined[col].values

Where the limitation lives

_validate_col_for_column_table() in src/spatialdata_plot/pl/utils.py (around line 2850) only checks:

col_for_color not in sdata[table_name].obs.columns and col_for_color not in sdata[table_name].var_names

No .obsm lookup is attempted.

Suggested behavior

Allow color="obsm_key:column" or similar syntax to reference a column within an .obsm DataFrame/array.