TUI: mouse drag selection with OSC 52 copy; Markdown emphasis follows CommonMark; v0.5.0

- Drag, double-click (word) and triple-click (line) selection in the log,
  auto-scroll at the edges, soft-wrapped lines joined on copy.
- sashidown ANSI renderer: delimiter runs with flanking rules, so
  intraword underscores (test_apply_patch.c) stay literal; unmatched
  * _ and backticks are shown as-is; * and _ do not close each other.
- tests: test_textsel, test_markdown.

include/cst_textsel.h

@@ -0,0 +1,41 @@
+#ifndef CST_TEXTSEL_H
+#define CST_TEXTSEL_H
+
+#include <stddef.h>
+#include <wchar.h>
+
+/* UTF-8 / ANSI helpers shared by the TUI renderer and mouse selection, so both
+ * agree on which character occupies each column. */
+int cst_utf8_decode(const char *s, size_t avail, wchar_t *wc, size_t *eaten);
+/* wcwidth(), with an East-Asian / ASCII heuristic on Windows. */
+int cst_wcwidth(wchar_t wc);
+/* Columns of wc; 1 for non-printing characters. */
+int cst_utf8_width(wchar_t wc);
+/* Length of the escape sequence at s (CSI or two-byte ESC), 0 if none. */
+size_t cst_ansi_skip(const char *s, size_t n);
+
+typedef struct {
+  char *s; /* NUL-terminated once anything was appended */
+  size_t n, cap;
+} cst_selbuf_t;
+
+void cst_selbuf_free(cst_selbuf_t *b);
+int cst_selbuf_putc(cst_selbuf_t *b, char c);
+
+/* Appends the text drawn in columns [c1, c2] of a row whose content `line`
+ * (ANSI escapes allowed) starts at column `indent`; c2 < 0 means the end of
+ * the row. A wide character is taken whole when either of its cells is in
+ * range. With `trim`, trailing spaces of the appended part are dropped.
+ * Returns 0, or -1 when out of memory. */
+int cst_textsel_append(cst_selbuf_t *b, const char *line, int indent, int c1,
+                       int c2, int trim);
+
+/* Column span [*c1, *c2] of the word under column `col`; just that cell (or
+ * both cells of a wide character) when it is not on a word character. */
+void cst_textsel_word(const char *line, int indent, int col, int *c1, int *c2);
+
+/* Widens [*c1, *c2] so that neither end splits a wide character; highlighting
+ * half of one desynchronizes curses from the terminal. *c2 < 0 is left as is. */
+void cst_textsel_snap(const char *line, int indent, int *c1, int *c2);
+
+#endif