lib/log: document format param as permanent (post-#19)

c9bbfcb's commit message floated a follow-up — "lib/log can revert
format→fmt now that the silent crash is impossible." That note was
wrong. #19's rule is decl-site and body-blind by design (single
rule, no non-local reasoning), so any `fn x(fmt: str)` under
`use fmt;` is refused regardless of whether the body calls fmt.X.
Rename infeasible.

Rewrite the header bullet to drop the wishlist sentence and state
the constraint directly: ww's `.` overload for both module-access
and field-access makes `use fmt; fn x(fmt: T)` structurally
ambiguous, and #19 refuses it at decl. The `format` parameter name
stays.

Comment-only; no test surface.

lib/log/log.ww

@@ -37,10 +37,11 @@
 //     some lib/log fn first so init runs.
 //
 //   • Param name divergence from Hare: the format-string parameter is
-//     `format` (not Hare's `fmt`) because ww's resolver shadows the
-//     `fmt` module reference inside a body when a same-name param is
-//     in scope (silent miscompile — CALL through str.ptr). Tracked as
-//     task #19; rename to `fmt` when fixed.
+//     `format` (Hare's is `fmt`). Permanent — #19 refuses any
+//     let/param that shadows an imported module name, regardless of
+//     body usage. ww chose `.` for both module-access and field-
+//     access, so `use fmt; fn x(fmt: T)` is structurally ambiguous;
+//     the resolver refuses it at decl.
 //
 // Sink today is [[io.stream]] only — lib/io has no fd-backed stream
 // yet (Hare's `io::handle = file | int` collapses to one variant).