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

lib/tcl/tests/idioms.sx

@@ -187,6 +187,57 @@
           (env-extend (env-extend base "a" 3) "b" 7)
           (quote (* a b))))
       21)
+
+    ; 26-32. Phase 5 channels: write/read/seek/tell/eof/append/non-blocking
+    (ok "channel-write-read"
+      (get
+        (run
+          "set f /tmp/tcl-phase5-1.txt\nset c [open $f w]\nputs $c \"line one\"\nputs $c \"line two\"\nclose $c\nset c [open $f r]\nset out [read $c]\nclose $c\nfile delete $f\nreturn $out")
+        :result)
+      "line one\nline two\n")
+
+    (ok "channel-gets-loop"
+      (get
+        (run
+          "set f /tmp/tcl-phase5-2.txt\nset c [open $f w]\nputs $c apple\nputs $c banana\nputs $c cherry\nclose $c\nset c [open $f r]\nset out {}\nwhile {[gets $c line] >= 0} {lappend out $line}\nclose $c\nfile delete $f\nreturn $out")
+        :result)
+      "apple banana cherry")
+
+    (ok "channel-seek-tell"
+      (get
+        (run
+          "set f /tmp/tcl-phase5-3.txt\nset c [open $f w]\nputs -nonewline $c \"hello world\"\nclose $c\nset c [open $f r]\nseek $c 6\nset pos [tell $c]\nset rest [read $c]\nclose $c\nfile delete $f\nreturn \"$pos:$rest\"")
+        :result)
+      "6:world")
+
+    (ok "channel-eof-after-read"
+      (get
+        (run
+          "set f /tmp/tcl-phase5-4.txt\nset c [open $f w]\nputs -nonewline $c hi\nclose $c\nset c [open $f r]\nread $c\nset e [eof $c]\nclose $c\nfile delete $f\nreturn $e")
+        :result)
+      "1")
+
+    (ok "channel-append-mode"
+      (get
+        (run
+          "set f /tmp/tcl-phase5-5.txt\nset c [open $f w]\nputs -nonewline $c \"first\"\nclose $c\nset c [open $f a]\nputs -nonewline $c \"-second\"\nclose $c\nset c [open $f r]\nset out [read $c]\nclose $c\nfile delete $f\nreturn $out")
+        :result)
+      "first-second")
+
+    (ok "channel-seek-end"
+      (get
+        (run
+          "set f /tmp/tcl-phase5-6.txt\nset c [open $f w]\nputs -nonewline $c \"abcdefghij\"\nclose $c\nset c [open $f r]\nseek $c 0 end\nset pos [tell $c]\nclose $c\nfile delete $f\nreturn $pos")
+        :result)
+      "10")
+
+    (ok "channel-fconfigure-blocking"
+      (get
+        (run
+          "set f /tmp/tcl-phase5-7.txt\nset c [open $f w]\nputs -nonewline $c x\nclose $c\nset c [open $f r]\nfconfigure $c -blocking 0\nset b [fconfigure $c -blocking]\nclose $c\nfile delete $f\nreturn $b")
+        :result)
+      "0")
+
     (dict
       "passed"
       tcl-idiom-pass