feat: rename log macros to debug!/info!/warn!/error! + add trace level and spans

[trunkszi]

Summary

• Rename level macros to align with the Rust logging ecosystem: logd!/logi!/logw!/loge! → debug!/info!/warn!/error!
• Add Trace severity (Level::TRC = 0) and a trace! macro; existing levels shift +1 (DBG=1, INF=2, WRN=3, ERR=4, OFF=5)
• Add a lightweight span subsystem (Span, SpanGuard, SpanId, Instrument future wrapper) with the span! macro and a {span} header pattern token

Breaking changes

• All logd!/logi!/logw!/loge! call sites must migrate to debug!/info!/warn!/error!. No deprecated aliases retained.
• Level numeric repr shifts: persisted/serialized level bytes from prior builds are no longer compatible (ferrilog has no on-disk Level serialization today, so this is internal-only).
• StaticHeaderWriteFn signature gains a trailing u64 (span_id). Any custom static header writer must add the parameter.

Implementation notes

• Payload layout grows by 8 bytes: [info_ptr][timestamp][span_id][encoded args...]
• Argument-order decision: span_id is placed at the end of write_record / write_header signatures. An earlier in-middle placement caused 60–140% regressions on the header_output benches because AAPCS64 spilled &str.len to the stack. Moving span_id last restores all benches to noise threshold and leaves thread_name/location in argument registers.

Test plan

• cargo check --workspace --all-features --all-targets
• cargo test --workspace --all-features — 71 tests pass, includes new span::tests::enter_and_drop_restore_current_span, span::tests::span_records_parent_id, header::tests::span_header_token_renders_dash_for_root_and_decimal_for_span
• cargo clippy --workspace --all-features --all-targets -- -D warnings — zero warnings
• cargo bench (latency_throughput, core_component_bench, ferrilog_bench) — 0 regressed, 1 improved, rest within noise; some hot-path cases ~6% faster than the pre-span baseline
• Manual span smoke test: nested spans render span=- → span=1 → span=2 → span=1 → span=- correctly
• Examples 01/02/04/07 run with expected output