tcl: Phase 5b event loop — fileevent/after/vwait/update

New SX primitive io-select-channels(read-list write-list timeout-ms) wrapping
Unix.select on the registered channel table. Returns {:readable :writable}.

Tcl event loop implemented purely in Tcl (no sx_server.ml changes):
- fileevent $chan readable|writable script (or "" to unregister)
- fileevent $chan event (1 arg) returns the registered script
- after ms script — schedule one-shot timer
- after ms (no script) — sleep, driving event loop in the meantime
- vwait varname — block until var is set/changed, handlers run between polls
- update — non-blocking event drain (poll-timeout=0)

State on interp: :fileevents (list of (chan event script)) and :timers
(sorted list of (expiry-ms script)).

tcl-event-step is the inner loop: expire timers, build fd lists from
:fileevents, call io-select-channels with computed timeout, run ready
handlers. vwait polls every 1000ms or until var changes.

Scoped to script mode by design — vwait from inside a server-handled
command does not interact with sx_server's stdin scheduler.

+5 idiom tests: after-vwait-timer, after-multiple-timers-update,
fileevent-readable-fires, fileevent-query-script,
after-cancel-via-vwait-timing. 354/354 green.

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

lib/tcl/tests/idioms.sx

@@ -238,6 +238,42 @@
         :result)
       "0")
 
+    ; 33-37. Phase 5b event loop: after / vwait / fileevent / update
+    (ok "after-vwait-timer"
+      (get
+        (run
+          "after 30 {set ::done fired}\nvwait ::done\nset ::done")
+        :result)
+      "fired")
+
+    (ok "after-multiple-timers-update"
+      (get
+        (run
+          "set ::n 0\nafter 0 {incr ::n}\nafter 0 {incr ::n}\nafter 0 {incr ::n}\nupdate\nset ::n")
+        :result)
+      "3")
+
+    (ok "fileevent-readable-fires"
+      (get
+        (run
+          "set f /tmp/tcl-phase5b-1.txt\nset c [open $f w]\nputs -nonewline $c hi\nclose $c\nset c [open $f r]\nfileevent $c readable {set ::ready 1; fileevent $::ch readable {}}\nset ::ch $c\nvwait ::ready\nclose $c\nfile delete $f\nset ::ready")
+        :result)
+      "1")
+
+    (ok "fileevent-query-script"
+      (get
+        (run
+          "set f /tmp/tcl-phase5b-2.txt\nset c [open $f w]\nputs -nonewline $c x\nclose $c\nset c [open $f r]\nfileevent $c readable {puts hello}\nset s [fileevent $c readable]\nclose $c\nfile delete $f\nreturn $s")
+        :result)
+      "puts hello")
+
+    (ok "after-cancel-via-vwait-timing"
+      (get
+        (run
+          "set ::counter 0\nafter 10 {incr ::counter}\nafter 50 {set ::done 1}\nvwait ::done\nset ::counter")
+        :result)
+      "1")
+
     (dict
       "passed"
       tcl-idiom-pass