Fixes for M-02 to M-14, G-01 to G-14, K-02 to K-04, X-04 to X-14, not yet reviewed

Committed before review so Windows and WSL builds can start.
Each fix was tested and mutation-checked here; the codex review
of each item is still to come, so tasks/BUGS.md stays unticked.

M-02 is mid-review: round 1 findings are fixed (apps/ely-mark/src/tally.rs).
K-01 stays deferred. The ElyMark window test mark-outside.sh failed
once at its crash-recovery step in the last run; not yet looked into.

crates/ely-platform/src/media.rs

@@ -5,6 +5,9 @@ use std::path::Path;
 #[cfg(target_os = "macos")]
 mod macos;
 
+/// Only macOS plays yet; elsewhere nothing offers it.
+pub const BUILT: bool = cfg!(target_os = "macos");
+
 /// What the player knows of its file.
 #[derive(Clone, Debug, PartialEq)]
 pub enum State {