fix: swap Too many/Too few messages and correct expected/have in callback arg errors (#42)

[KorsarOfficial]

Problem

The two error messages for callback argument count mismatches in check-func-calls-and-vararg.cpp had their labels swapped and the expected/have format arguments transposed.

Formal Analysis

Define argument validation predicates:

n   = actual argument count    (call_n_params)
min = minimum required args    (f->get_min_argn())
max = maximum allowed args     (f->get_params().size())

P_min(n) := n >= min    (sufficient arguments)
P_max(n) := n <= max    (not excessive arguments)

Error mapping:
  not P_min(n) => "Too few arguments"    (n < min)
  not P_max(n) => "Too many arguments"   (n > max)

Bug — two compound errors:

Condition	Old (wrong)	New (correct)
n < min (too few)	"Too many, exp=n, have=min"	"Too few, exp=min, have=n"
n > max (too many)	"Too few, exp=n, have=max"	"Too many, exp=max, have=n"

Truth table (min=2, max=4):

n	Condition	Old output	Correct output
1	n < min	"Too many, exp=1, have=2"	"Too few, exp=2, have=1"
3	valid	(no error)	(no error)
5	n > max	"Too few, exp=5, have=4"	"Too many, exp=4, have=5"

Fix

Swap message strings and reorder fmt_format arguments to match the non-callback pattern on lines 271-274 (known correct reference).

Verification: After fix, callback error structure is isomorphic to the non-callback pattern:

non-callback:  kphp_error(n >= min, "Too few ..., exp=min, have=n")
callback:      kphp_error(n >= min, "Too few ..., exp=min-d, have=n-d")
                                                    ^^^^^^^^^^^^^^^^^^^^
                                              identical modulo delta offset

∎

Technical Report

Full report with mathematical formalization (PDF)

Closes #42