status: name the open file

The status bar said "0x2490" without saying what it belonged to. Obvious once
there are two panes open, or after switching binaries in a project — which
already prefixed its label, so single-binary sessions were the odd ones out.

  [cat] .text @ 0x472b   [listing]   (c code · p func · u undefine · Enter follow)

Uses the opened file's basename, not _module(): that one asks the worker over
RPC and this runs on every status write. Kept in step when the path changes
(project switch, reload).

Three messages already carried the module name themselves and would have read
"[echo] echo — 128 functions"; they don't say it twice now.

tests: +3 scenarios (212) — the bar names the file, keeps naming it as you move
(the idle status is not the only writer), and doesn't say it twice.

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