tkinter façade: a callable option that receives Tk's OWN arguments

bind and -command hand a Python callable either nothing or one Event, and the dispatcher (pxxcb) covers both. Tk also calls options with its own argument lists, and those have no path through the façade yet:

For the scroll pair this does not matter today: CPython's tkinter does not call back into Python for them either — it wires Tcl straight to the other widget's subcommand, and TkiOptScrollCmd now does the same (receiver → widget path, option name → subcommand). Anything that is NOT a widget method is refused loudly there rather than wired to something wrong.

What is missing is the general shape:

1. a registry entry that records "this callback takes Tcl's raw arguments";
2. a dispatcher branch that packs argv[2..] into Python values instead of building an Event;
3. call bridges for 2 and 3 arguments — pycallback_call2/3 for a bound method, and a bound-fn / closure that accepts more than one own parameter (today pyboundfn_call_ptr passes exactly one).

Until it lands, a plain def or lambda as a scroll command fails with a message naming the limit.

Why it was filed

songformatter's settings.py writes both canonical spellings (Scrollbar(..., command=self.canvas.yview) and canvas.configure(yscrollcommand=self.scrollbar.set)). Passing a bound method into the façade's former AnsiString option compiled silently and handed Tcl a garbage script; the event loop then hung inside update. See [[bug-nilpy-bound-method-coerced-to-string]].

Moved to blocked/ 2026-07-31 — the missing piece is runtime, not façade

Re-read against the code rather than the ticket text. Of the three things listed under "what is missing", the first two are façade work Track B owns — a registry flag and a dispatcher branch. The third is not:

call bridges for 2 and 3 arguments — pycallback_call2/3 for a bound method, and a bound-fn / closure that accepts more than one own parameter (today pyboundfn_call_ptr passes exactly one).

pycallback_call0/1 live in compiler/builtin/pylib.pas:581 and pyboundfn_call_ptr in pyeval.pas:1870 — shared NilPy runtime, which Track B does not edit. Without them the façade half has nothing to call, so building the registry and the dispatcher branch first would land dead code.

Split and filed as [[feature-nilpy-multi-arg-callback-bridges]] (Track N). Tagged for later: when that lands, the remaining work here is small and entirely in lib/pcl/tkinter.pas.

Nothing regresses in the meantime. The case that motivated this ticket — the scrollbar pair — turned out not to need it at all: CPython's tkinter does not call back into Python for yscrollcommand either, it wires Tcl straight to the other widget's subcommand, and TkiOptScrollCmd now does the same and refuses loudly for anything that is not a widget method. Nothing enters the regression suite from here until the bridges exist.

2026-08-03 — dependency recorded in frontmatter

The 2026-07-31 note above identified the blocker in prose (the shared-runtime pycallback_call2/3 / multi-arg pyboundfn_call_ptr bridges) but no blocked-by: edge existed, so the board could not see it: this ticket ranked as ready and would have been handed to a Track B agent who cannot edit compiler/builtin/pylib.pas. The bridges have their own ticket — [[feature-nilpy-multi-arg-callback-bridges]] — and it is now the recorded edge, so priority propagates to it and this surfaces only once it lands.