tcl: Phase 5 channel I/O — open/read/gets/puts/seek/tell/eof/fconfigure

11 new SX primitives in sx_primitives.ml wrapping Unix.openfile/read/write/
lseek/set_nonblock: channel-open/close/read/read-line/write/flush/seek/tell/
eof?/blocking?/set-blocking!.

Tcl runtime now uses real channel ops:
- open ?-mode? returns "fileN" handle (modes r/w/a/r+/w+/a+)
- close/read/gets/puts/seek/tell/eof/flush wired through
- new fconfigure command supports -blocking 0|1
- puts dispatches to channel-write when first arg starts with "file"
- gets command registration fixed (was pointing to old stub)

eof-returns-1 coro test updated to match real Tcl semantics (eof flips
only after a read hits EOF).

Test runner timeout bumped 180s→1200s (post-merge JIT is slow).

+7 idiom tests covering write+read, gets-loop, seek/tell, eof-after-read,
append mode, seek-to-end, fconfigure-blocking. 349/349 green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/tcl/tests/coro.sx

@@ -124,7 +124,7 @@
       "file0")
 
     (ok "eof-returns-1"
-      (get (run "set ch [open /dev/null r]\neof $ch") :result)
+      (get (run "set ch [open /dev/null r]\nread $ch\neof $ch") :result)
       "1")
 
     (dict