[audio] AudioFormatNegotiator::primaryRateOrder: add Q_UNREACHABLE on switch fall-through

[ten9876]

Tiny follow-up to #3397 (merged as 204585b).

AudioFormatNegotiator::primaryRateOrder() (src/core/AudioFormatNegotiator.cpp:19-47) has two per-direction switches over TargetOs, then a return {internalRate} fallback after both switches. The fallback is only reachable if a new TargetOs enum value is added later without updating one of the inner switches.

Flagged in the @aethersdr-agent bot review as a non-blocking nit. The current shape is correct; this is documentation polish for future contributors.

Suggested change

Replace the trailing fallback with Q_UNREACHABLE() so the invariant ("every TargetOs value is handled in both directions") is enforced by the compiler / runtime instead of papered over with a silent default:

QList<int> primaryRateOrder(TargetOs os, Direction dir, int internalRate)
{
    if (dir == Direction::Output) {
        switch (os) {
        case TargetOs::Windows: return {48000, internalRate};
        case TargetOs::MacOS:   return {48000, internalRate};
        case TargetOs::Linux:   return {internalRate, 48000};
        }
    } else {
        switch (os) {
        case TargetOs::Windows: return {48000, 44100, internalRate, 16000};
        case TargetOs::MacOS:   return {48000, 44100, internalRate, 16000};
        case TargetOs::Linux:   return {internalRate, 48000, 44100};
        }
    }
    Q_UNREACHABLE();  // every TargetOs value is covered above
}

When

Cheap enough to roll into the next audio-sink-factory PR (the live Qt wrapper, currently open as #3398) rather than landing as a standalone change.

Refs #3306 #3397

[aethersdr-agent]

Confirmed against main (204585b).

Analysis

src/core/AudioFormatNegotiator.cpp:19-47 has the exact shape described: two switch (os) blocks (one per Direction) covering all three TargetOs values (Windows, MacOS, Linux — src/core/AudioFormatNegotiator.h:38), then a trailing return {internalRate}; on line 46.

Today every TargetOs enumerator is handled in both switches, so line 46 is unreachable. It only becomes live if a fourth TargetOs value (say TargetOs::FreeBSD) gets added later without updating both switches — in which case the silent {internalRate} fallback would mask the omission instead of failing loudly. Q_UNREACHABLE() flips that to a compiler hint + runtime assert.

Worth noting: Q_UNREACHABLE is not currently used anywhere else in the codebase. That's not a blocker — it's a standard Qt macro — but the maintainer may want to consider whether to standardize on it for the other exhaustive-switch sites in this same file (toString(SampleFmt) at line 202-209, toString(ResamplerKind) at 211-219, toString(TargetOs) at 221-229, resamplerKindFor at 70-79) for consistency, or apply it only here.

Proposed fix

Single-file change in src/core/AudioFormatNegotiator.cpp:

• Replace the return {internalRate}; on line 46 with Q_UNREACHABLE();
• Optionally add a short trailing comment as the issue suggests (// every TargetOs value is covered above)

No header change needed (Q_UNREACHABLE is available via <QtGlobal>, already transitively included through Qt headers used in AudioFormatNegotiator.h).

Handoff

• Known: location confirmed, fix is one line, no behavioral change today.
• To decide: scope — just primaryRateOrder, or sweep the other exhaustive switch sites in the same file too? The issue scopes it to just primaryRateOrder.
• When: issue author suggests bundling into the next audio-sink-factory PR (feat(audio): live wrapper + RX & TX-mic migration onto the sink factory (#3306) #3398) rather than a standalone change. Reasonable — but if feat(audio): live wrapper + RX & TX-mic migration onto the sink factory (#3306) #3398 stays open a while, a small standalone PR is also fine since the change is self-contained and trivially reviewable.
• Test: existing tests/audio_format_negotiation_test.cpp already covers all three TargetOs × Direction combinations, so no new test needed; CI will catch any accidental fallthrough.

Flagging good first issue — it's a single-line change with a clear spec and existing test coverage.

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