perf(ui/connection_table): borrow instead of clone in Location, Service, and Application cells

[obchain]

Problem

Three cells in the per-connection-row render path allocate Strings that could instead borrow from the existing data.

1. Location column — country_display().to_string() (line 382)

let location = conn
    .geoip_info
    .as_ref()
    .map(|g| g.country_display().to_string())   // allocates "US", "DE", …
    .unwrap_or_else(|| NONE_PLACEHOLDER.to_string()); // allocates "-"
Cell::from(location)

country_display() already returns &str (borrowed from the inner country_code). The .to_string() call and the NONE_PLACEHOLDER.to_string() fallback both allocate a String only to pass it to Cell::from. Since connection_row annotates conn: &'a Connection, the borrow can live long enough.

2. Service column — service_name.clone() in service_text (line 204–207)

fn service_text(conn: &Connection, ui_state: &UIState) -> String {
    if ui_state.show_port_numbers {
        conn.remote_addr.port().to_string()
    } else {
        conn.service_name
            .clone()                                  // clones the Option<String>
            .unwrap_or_else(|| NONE_PLACEHOLDER.to_string())  // allocates fallback
    }
}

The return value is immediately passed as &str to truncate_with_ellipsis. conn.service_name is Option<String>; we can borrow the inner &str via as_deref() instead of cloning. Changing the return type to Cow<'a, str> allows:

• Cow::Borrowed(name) when a service name is present — zero allocation
• Cow::Borrowed(NONE_PLACEHOLDER) when absent — zero allocation
• Cow::Owned(port.to_string()) for the port-number branch — same as before

3. Application column compact branch — sort_key().to_string() (line 439)

let app = if width >= APP_WIDTH_FULL {
    dpi.application.to_string()
} else {
    dpi.application.sort_key().to_string()  // &'static str → String → &str
};
let budget = …;
let app = truncate_with_ellipsis(&app, budget);

sort_key() returns &'static str. Computing budget first and passing sort_key() directly to truncate_with_ellipsis eliminates the intermediate String.

Fix

• Add use std::borrow::Cow import.
• Annotate conn: &'a Connection in connection_row so borrowed cells get the right lifetime.
• Location cell: .map(|g| g.country_display()).unwrap_or(NONE_PLACEHOLDER) → Cell::from(&str).
• service_text → Cow<'a, str> with as_deref() + Cow::Borrowed.
• Application compact branch: compute budget first, call truncate_with_ellipsis on sort_key() directly.

Impact

Per connection row per frame:

• Location cell: −1 String allocation (borrow from GeoIpInfo or static)
• Service cell (service-name branch): −1 String clone + −1 String allocation for the placeholder case
• Application cell (compact branch, common case): −1 intermediate String allocation