Mermaid pie legend: extreme values round wrong via f64->i64 cast #12

Summary: trim_num in crates/mermaid/src/pie.rs:184-190 casts an f64 legend value to i64 when the fractional part is zero. For values at or above 2^63 (~9.22 quintillion) with zero fract, the cast saturates and the legend displays 9223372036854775807 instead of the actual (much larger) value.

Severity: S4 — latent/internal. No panic, no data corruption, and a pie-chart value of ≥2^63 is an extreme, unlikely-in-practice input, but it is a genuine silent-wrong-display defect rather than a graceful rejection or clamp-with-warning.

Environment:

• Deploy: any (pure function, no environment dependency)
• Commit: 6411ddb

Repro steps:

1. Call trim_num(9223372036854775808.0) (i.e. 2f64.powi(63), which has zero fractional part).
2. Observe the returned string.

Expected: Either a value that round-trips reasonably close to the input, or an explicit "value too large to display precisely" fallback.

Observed: Displays 9223372036854775807 (i64::MAX), silently wrong by construction of the as i64 saturating cast.

Evidence:

• Code: crates/mermaid/src/pie.rs:184-190 (trim_num), the v as i64 cast on the zero-fract branch.
• Existing test trim_num_formats_whole_and_fractional_values (crates/mermaid/src/pie.rs:378-381) only covers small/ordinary values (7.0, 0.0, 1.5), not the overflow boundary.

Suspected area: crates/mermaid/src/pie.rs::trim_num — needs a saturating-cast-aware guard or a different formatting strategy for values outside i64's exact range.