fix(logstats): resolve config features and mission defaults

[gbin]

Summary

• Load conditional Copper config fragments using log-stats CLI features.
• Prefer the mission recorded in the unified log, then the named default mission, then a stable first mission.
• Update the public API baseline for the additive features argument.

Stack

PR 1 of 4.

• Next: feat(logstats): export observed schedule timing #1250
• Then: feat(plan): render observed execution timing #1251
• Final: feat(dx): add plan-log workflows #1245

Review and merge from this PR upward.

Verification

• Rust formatting check passed for all changed Rust files.
• Diff whitespace check passed.
• Full workspace verification is deferred to CI because the local checkout is missing the optional sibling cu-vitfly path dependency.