fix: line-buffer stdout when redirected so progress/readiness lines flush

When crafter-build's stdout is not a TTY (redirected to a file or pipe) the
C runtime defaults to full (block) buffering. The progress path is TTY-aware
and the in-place redraw flushes explicitly, but the non-TTY append path
(`[N/M]` lines), Finalize()'s `Built N steps` line and the `-r` server's
`listening on port :N` line all go through block-buffered stdout with no
flush. They accumulate in the buffer and only spill at ~4KB boundaries.

On a normal build this is hidden because the C runtime flushes stdout at
exit. Under `-r` the process never exits — it blocks in its serve loop — so
the trailing buffer is never flushed: a redirected log freezes mid-build (or
sits at 0 bytes) even though the build finished and the server is already
answering. Any tooling that polls the log for `Built …` / `listening …` /
`[N/N]` hangs forever. This is the real cause of the frozen log misdiagnosed
as a build deadlock in #16.

Fix: switch stdout to line buffering at the very top of main(), before any
output, only when stdout is not a terminal. Every `\n` then flushes, so the
markers reach a redirected log immediately. No behaviour change on a TTY.

Kept self-contained in main.cpp using system headers (isatty + setvbuf)
rather than a new Crafter::Progress export: the self-hosting exe build
compiles main.cpp against the installed/cached Crafter.Build module BMIs,
which shadow the freshly built local ones, so a new interface symbol would
not be visible without reinstalling crafter-build first.

Resolves #18

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

implementations/main.cpp

@@ -17,9 +17,33 @@ License along with this library; if not, write to the Free Software
 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301 USA
 */
 
+#include <cstdio>
+#if defined(_WIN32)
+    #include <io.h>
+    #ifndef STDOUT_FILENO
+        #define STDOUT_FILENO _fileno(stdout)
+    #endif
+    #define CRAFTER_MAIN_ISATTY _isatty
+#else
+    #include <unistd.h>
+    #define CRAFTER_MAIN_ISATTY isatty
+#endif
+
 import std;
 import Crafter.Build;
 
 int main(int argc, char** argv) {
+    // Line-buffer stdout when it is not a terminal. Must run before any stdout
+    // write. When stdout is a regular file or pipe the C runtime defaults to
+    // full (block) buffering, so the append-only progress lines ("[N/M]") and
+    // the trailing "Built … steps" / "listening on port …" lines would sit in
+    // the buffer and never reach a redirected log — fatal for tooling that
+    // polls the log for a readiness marker, and worst under -r where the
+    // process blocks in its serve loop and never returns to flush. A TTY is
+    // already line-buffered and its in-place redraw path flushes explicitly, so
+    // only adjust when redirected.
+    if (CRAFTER_MAIN_ISATTY(STDOUT_FILENO) == 0) {
+        std::setvbuf(stdout, nullptr, _IOLBF, 0);
+    }
     return Crafter::Run(argc, argv);
 }