Investigate feasibility of new CLI features: result export, baselining, CI regression testing

[Copilot]

Investigation for #222: assess the feasibility of adding result export, baselining, and CI regression testing of query performance to the sqlstresscmd CLI. This PR adds a planning document only — no runtime behaviour changes.

Added

• docs/cli-features-feasibility.md — feasibility plan grounded in the current code (Program.cs → LoadRunner → shared LoadEngine).

Key findings

• Result export already partially exists: the -r/--results option autosaves a CSV in LoadRunner, though the option is still annotated //TODO Implement and only emits averages with culture-sensitive formatting.
• Enabling changes that unlock all three features are small: a shared RunResult model for structured export, and making Program.Main return a non-zero exit code (currently void, so CI cannot detect failures).
• The CLI already collects the needed metrics and reuses the linked JsonSerializer and Spectre.Console, so no new heavy dependencies are required.

Proposed design & effort

• Result export (Low) — formalize RunResult, add JSON, invariant/ISO-8601 formatting, optional percentiles.
• Baselining (Low–Medium) — --save-baseline / --baseline comparing per-iteration averages, rendered as a diff table.
• CI regression testing (Medium) — threshold options + exit codes + sample GitHub Actions job; flagged as noise-sensitive, so recommended as indicative rather than exact.

Recommended delivery order

Export → baselining → CI regression, each additive and backward compatible.

• Fixes Investigate new CLI features #222