Document snapshot pointer lifetimes and make getters NULL-safe

The snapshot getters crashed on a NULL snapshot, unlike the rest of the
API. They now return NULL, an empty span, 0, or write nothing, as the
terminal functions do. term_snapshot_cluster writes nothing when cps is
NULL or cap is 0.

Document that the view and its palette keep their address for the life
of the snapshot, and that a row pointer lasts only until the next update
or free, since an update can realloc the cell array.

Rename the term_input.h include guard to TERM_INPUT_H to match the file.

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

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