Structured key-value fields

How log::kv structured pairs surface as DLT verbose arguments so DLT Viewer / dlt-tui can index on them, rather than being collapsed into the formatted message (a refinement of the DLT backend, taktora-log-dlt DLT-protoco... (FEAT_0072)).

Feature: Structured key-value fields mapped to DLT verbose args FEAT_0074
status: approved
satisfies: FEAT_0072
is satisfied by: REQ_0809

The log crate (v0.4.21+) supports structured key-value pairs alongside the formatted message. taktora-log-dlt shall surface those pairs as DLT verbose arguments rather than collapsing them into the formatted message, so DLT Viewer / dlt-tui can index on them.

Requirement: log::kv pairs encoded as DLT verbose arguments REQ_0809
status: implemented
satisfies: FEAT_0074
is verified by: TEST_0809
links outgoing: BB_0091, TEST_0809

For each log::Record, the DLT backend shall iterate record.key_values() and emit one DLT verbose argument per structured pair. Native mappings shall apply where the value’s type matches a DLT primitive: u32 / i32 / u64 / i64 / f32 / f64 / bool / &str. Values whose type does not have a native DLT mapping shall be rendered via Display and encoded as a verbose string argument. The formatted message body shall always be emitted as the leading argument regardless of structured-field presence.