Tryst

The Tcl-major-version auto-detection probe (TCL_VERSION=8/9 forces a choice; anything else auto-detects - see interp.cr's header comment for the full policy and why this needs to be a shell probe at all) used to be hand-copied verbatim at every {% if %}/{% unless %} site that needs it: twice in interp.cr, once in event_source.cr. Crystal macro locals don't persist across separate top-level {% %} blocks (confirmed directly - a {% x = ... %} in one block is invisible to the next), so sharing a computed result isn't an option. A plain Crystal constant doesn't have that limitation, though, and a macro backtick literal accepts #{...} interpolation of one directly - `#{X.id}` splices X's raw text into the backtick before it runs - so the SCRIPT TEXT lives here exactly once, and every call site becomes an identical one-liner: `#{Tryst::TCL_VERSION_PROBE.id}`.stringify.chomp == "9" (still re-executed at every site - that part of the duplication is a real Crystal limitation, not a style choice - just no longer re-typed, so a bug in the probe itself only needs fixing once).

Two variants, chosen once here rather than at every call site, for the same reason expand_lib_flags's Windows @[Link] ldflags need their own variant (see tcltk_link_windows.cr's header comment for the full explanation): a macro backtick literal evaluates through the same Process.run(shell: true) that never goes through an actual shell on Windows, so the POSIX case/if script below has to be handed to a real sh.exe explicitly there, with pkg-config resolved relative to the running crystal binary rather than trusted from PATH search order (a stale, unrelated MSYS/Cygwin install earlier in Machine PATH will otherwise silently win and misreport - confirmed directly).