fix(d_net): silence static_mut_refs on EXITMSG buffer

[sunsided]

Summary

EXITMSG.len() inside PlayerQuitGame (room/src/doom/d_net.rs:302) implicitly created a shared reference to the function-local static mut EXITMSG just to read its array length, tripping the Rust 2024 static_mut_refs warning that ships as a deny-by-default candidate in the 2024 edition.

The buffer length is a compile-time constant (80), so the lint is mechanically eliminated by hoisting the length into a local const.

Before

static mut EXITMSG: [c_char; 80] = [0; 80];

M_StringCopy(
    std::ptr::addr_of_mut!(EXITMSG[0]),
    DEH_String(EXIT_MSG.as_ptr() as *const c_char),
    EXITMSG.len(),
);

EXITMSG.len() requires &EXITMSG, which is a shared reference to a static mut. cargo check emits:

warning: creating a shared reference to mutable static
   --> room/src/doom/d_net.rs:302:9
    |
302 |         EXITMSG.len(),
    |         ^^^^^^^^^^^^^ shared reference to mutable static
    = note: `#[warn(static_mut_refs)]` (part of `#[warn(rust_2024_compatibility)]`) on by default

After

const EXITMSG_LEN: usize = 80;
static mut EXITMSG: [c_char; EXITMSG_LEN] = [0; EXITMSG_LEN];

M_StringCopy(
    std::ptr::addr_of_mut!(EXITMSG[0]),
    DEH_String(EXIT_MSG.as_ptr() as *const c_char),
    EXITMSG_LEN,
);

No reference to the static is taken. The array type still uses the same constant so the length stays single-sourced.

Outcome

• cargo check lib warnings drop from 1 to 0 (the static_mut_refs hit was the only one).
• cargo build passes.
• gitnexus_impact(PlayerQuitGame, upstream): LOW, 0 direct callers in the indexed graph (PlayerQuitGame is invoked from TryRunTics via direct call, not through a tracked edge).
• gitnexus_detect_changes: LOW, no execution flows affected.

Review Instructions

1. Read room/src/doom/d_net.rs:294-313 - confirm only the literal 80 was hoisted into EXITMSG_LEN and the offset arithmetic (EXITMSG[7] += player_idx as c_char;) is unchanged.
2. Verify the M_StringCopy size argument is still 80 (now expressed as EXITMSG_LEN).
3. Confirm no other call site reads EXITMSG.len() (it's a function-local static, scope is PlayerQuitGame only).

[Copilot]

Pull request overview

This PR removes a Rust 2024 static_mut_refs lint hit in PlayerQuitGame by avoiding an implicit shared reference to a function-local static mut buffer when passing its size to M_StringCopy.

Changes:

• Hoists the literal exit-message buffer length (80) into a local const EXITMSG_LEN.
• Replaces EXITMSG.len() with EXITMSG_LEN to avoid taking &EXITMSG (shared ref to static mut).
• Updates the EXITMSG array type and initializer to use the new constant.

---

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[sunsided]

Fixed in 6d324f0: the original buffer doc comment now sits directly above static mut EXITMSG, and EXITMSG_LEN got its own short doc explaining why the constant exists (so M_StringCopy doesn't need to take a shared reference to the static to read its length).