New config option for logging style

[josecelano]

Relates to: #878 (comment)

After migrating from the log to the tracing crate, we can now easily support multiple formats for logging.

@da2ce7 has proposed these 4 styles:

#[derive(Debug)]
pub enum TraceStyle {
    Default,
    Pretty(bool),
    Compact,
    Json,
}

That is already included in the code, but it's hardcoded to the TraceStyle::Default value. It's not possible to change it from the configuration.

@da2ce7 also proposes these changes in the TOML config file:

Current:

[logging]
threshold = "info"

New:

[logging]
trace_filter = "info"
trace_style = "full"

trace_filter replaces threshold option in the current version.

Note (2026-07-14): The original issue description referenced log_level as the current field name, but it was renamed to threshold in commit 287e484 (feat!: [#958] improve metadata in config files). The spec at docs/issues/drafts/configuration-overhaul-889-logging-style.md has been updated accordingly.

[josecelano]

Spec update (2026-07-15)

This issue has been incorporated into EPIC #1978 — Configuration Overhaul (schema v3.0.0) as subissue #8 of 9.

Spec: docs/issues/open/889-1978-new-config-option-for-logging-style.md

Key design decisions made

TraceStyle enum redesigned — the existing enum had two problems:

1. Default was a confusing name for a concrete style — renamed to Full
2. Pretty(bool) carried a display_filename boolean that is a cross-cutting option (applies to all styles, not just Pretty) and is rarely needed — dropped the boolean; Pretty is now a unit variant

New enum:

pub enum TraceStyle {
    Full,     // was Default — standard human-readable output (default)
    Pretty,   // was Pretty(false) — pretty-printed with colours
    Compact,  // compact single-line output
    Json,     // structured JSON output
}

display_filename defaults to false and can be added as a separate [logging] field later if users request it.

Config changes (config schema v3.0.0):

# Before
[logging]
threshold = "info"

# After
[logging]
trace_filter = "info"
trace_style = "full"

EPIC position: Independent — can run in parallel with #1415, #1453, #1490. No config overlap with other subissues.