tcl: Phase 6 coroutines + clock/file stubs + idiom corpus (+40 tests, 329 total)

- Coroutines (generator-style): coroutine/yield/yieldto commands; eager yield
  collection during body execution, pop-on-call dispatch via registered command
  closures; coro-yields + coroutines threaded through tcl-call-proc
- info exists varname (plus hostname/script/tclversion stubs)
- clock seconds/milliseconds/format/scan stubs
- File I/O stubs: open/close/read/eof/seek/tell/flush + file subcommands
- format command: full %-specifier parsing with flags, width, zero-pad, left-align
- Fixed dict set/unset/incr/append/update to use tcl-var-get (upvar alias aware)
- Fixed lappend and append to use tcl-var-get for reading (upvar alias aware)
- 20 coroutine tests (coro.sx) + 20 idiom corpus tests (idioms.sx)
- event-loop.tcl program: cooperative scheduler demo using coroutines
- Note: coroutines eagerly collect yields (generator-style, not true suspension)

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

lib/tcl/tests/programs/event-loop.tcl

@@ -0,0 +1,22 @@
+# expected: done
+# Cooperative scheduler demo using coroutines (generator style)
+# coroutine eagerly collects all yields; invoking the coroutine name pops values
+
+proc counter {n max} {
+    while {$n < $max} {
+        yield $n
+        incr n
+    }
+    yield done
+}
+
+coroutine gen1 counter 0 3
+
+# gen1 yields: 0 1 2 done
+set out {}
+for {set i 0} {$i < 4} {incr i} {
+    lappend out [gen1]
+}
+
+# last val is "done"
+lindex $out 3