[nicolas-abril]

Proposal 6 of #1632, in a smaller form than #1736.

The observed type already names what's missing, but it's easy to misread if you don't know that @b:U32 -> U32 means you still owe a b. This says so, and leaves out #1736's count of arguments given and taken:

SOME PROOFS FAIL
Error:
- expected : U32
- observed : @b:U32 -> U32
Location: main
 8 | def main() -> U32:
 9>|   add2(1)
   |   ^^^^^^^
10 | 
Note: this call is missing arguments.

• bend2/bend.ts: 4 lines at check-any's final throw, +102 ttok; with fix(check): a self-call that doesn't shrink shows what it received (#1632) #2006 and fix(check): a pair where Data is expected names the spelling that is (#1632) #2007, bend.ts is at 45,396 / 48k. prototype #1632.6: say how many arguments a partial call is missing #1736 adds about 485.
• It keeps prototype #1632.6: say how many arguments a partial call is missing #1736's guard: the observed type is a ∀, the expected type isn't, and the head is a Ref to a non-foreign def. Without the guard the note also fired on check/do_header_typed (a wrong header, where the do block passes the continuation) and check/neq_not_convertible (a variable), neither of which is missing an argument.
• A def that returns a function, used where a value is expected, gets the note too. The wording still holds, though the real bug may be elsewhere.
• tests/parse/glued_bracket.bend gains the note. tests/check/partial_call_note.bend pins it.

Touches the same throw as #2007; this PR is rebased on it and combines the two notes in one nte.

Gates on the three PRs combined, on origin/main 3ca3df32: repo 54/54, test 1819/1819, perf 124/124. Ping fails 80/81 on the BendHub LICENSE check, the same as unchanged main on this machine.

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