feat(types): add optional trit_signal() on Confidence

[eriirfos-eng]

Summary

Adds a trit_signal() method to the existing Confidence type that maps a
scalar confidence score to a ternary trit value from
ternlang-core.

Why

Pose estimation confidence often needs more than high/low. When the model
returns 0.48, forcing a binary yes/no decision discards the signal that this
reading is genuinely ambiguous and a human should review it.

The Tend trit provides exactly that: an explicit "I need more data" state.
This aligns with EU AI Act Article 14 human oversight requirements —
rather than guessing in the ambiguous range, the system defers.

use wifi_densepose_core::Confidence;

let c = Confidence::new(0.51)?;

#[cfg(feature = "ternlang")]
match c.trit_signal() {
    Trit::Affirm => { /* high confidence pose — act on it */ }
    Trit::Tend   => { /* uncertain — queue for human review */ }
    Trit::Reject => { /* low confidence — discard or retry */ }
}

Threshold mapping

Confidence range	Trit	Meaning
>= 0.65	Affirm	High confidence — proceed
0.35 .. 0.65	Tend	Uncertain — defer/review
< 0.35	Reject	Low confidence — discard

What changed

• src/types.rs: trit_signal() method added, feature-gated behind #[cfg(feature = "ternlang")]
• Cargo.toml: ternlang-core = "0.3" added as optional dependency; new ternlang feature flag
• All existing API is unchanged — Confidence(f32), new(), value(), is_high(), exceeds(), MAX, MIN are identical to upstream

Dependencies

ternlang-core = { version = "0.3", optional = true }

No local paths. Builds from crates.io on any machine.

[eriirfos-eng]

CI note: The ci.yml and security-scan.yml failures are pre-existing in this repo (same failures on 3b4e151 and earlier commits before this PR). The root cause is cargo test --workspace --no-default-features — the std feature is part of default in wifi-densepose-core, so disabling it breaks String/format! resolution.

Our changes compile cleanly:

• cargo check -p wifi-densepose-core → ✅
• cargo check -p wifi-densepose-core --features ternlang → ✅ (pulls ternlang-core 0.3 from crates.io)