feat: add --warmup auto to detect stable warmup count (fixes #848)

[leno23]

Summary

Fixes #848.

Adds support for --warmup auto, which keeps performing warmup runs until the relative spread of the last 5 warmup timings is at most 1%. Warmup stops early once stable, or after 100 runs at most.

Numeric --warmup NUM behavior is unchanged.

Changes

• Introduce WarmupOption (Disabled, Fixed, Auto) replacing the raw warmup_count
• Parse --warmup auto in CLI/options
• Refactor warmup execution in the benchmark runner and share iteration logic between fixed/auto modes
• Add unit tests for parsing/stability detection and an integration smoke test

Test plan

• cargo test warmup
• cargo test test_parse_warmup_option test_is_warmup_stable
• Full cargo test

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