Report selection allocation failure instead of truncating

On a screen taller than 256 rows the selection's row list is allocated.
When that failed, it fell back to the 256-entry stack array, so the
selection and the snapshot's selection spans silently stopped at row
256 while every call still reported success.

lines_init() now reports the failure. term_select_start() and
term_select_extend() return false and leave the selection unchanged,
term_snapshot_update() fails and keeps its previous content, and the
text getters return NULL, as they do for their other allocations.

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

1 files changed, 6 insertions, 4 deletions

diff --git a/include/term/term.h b/include/term/term.h
index c69e88e..8b98a25 100644
--- a/include/term/term.h
+++ b/include/term/term.h
@@ -336,11 +336,13 @@ enum term_select_unit {
 };
 
 /* Start a selection at cell. For TERM_SELECT_CELL, right_half means the
- * pointer is on the right half of the cell, so the selection starts after it. */
-TERM_API void term_select_start(struct term *t, struct term_vector2i cell, bool right_half,
+ * pointer is on the right half of the cell, so the selection starts after it.
+ * Returns false when memory runs out; the selection is then unchanged. */
+TERM_API bool term_select_start(struct term *t, struct term_vector2i cell, bool right_half,
                        enum term_select_unit unit);
-/* Extend the selection from its start to cell. */
-TERM_API void term_select_extend(struct term *t, struct term_vector2i cell, bool right_half);
+/* Extend the selection from its start to cell. Returns false when memory runs
+ * out; the selection is then unchanged. */
+TERM_API bool term_select_extend(struct term *t, struct term_vector2i cell, bool right_half);
 TERM_API void term_select_clear(struct term *t);
 TERM_API bool term_has_selection(const struct term *t);
 /* Owned, NUL-terminated UTF-8 of the selection. NULL when nothing is