perf(frequency): hint UTF-8 failure as cold in ignore-case hot loop

In `ftables_weighted_internal` and `ftables_unweighted`, the per-cell
`process_field` closures take an `if let Ok(s) = simdutf8::basic::from_utf8(field)`
branch where the `Ok` arm dominates on real data and the `Err` arm is rare.
Mark the four `else` arms with `core::hint::cold_path()` so LLVM keeps the hot
UTF-8-success path contiguous in the instruction cache.

Benchmark on a 1M-row NYC 311 CSV (514 MB, 41 cols), hyperfine, 10 runs:

  qsv frequency --ignore-case
    baseline 4.399 ± 0.045 s
    coldpath 4.139 ± 0.093 s   → 1.06× faster

  qsv frequency --ignore-case --no-trim
    baseline 4.089 ± 0.090 s
    coldpath 4.053 ± 0.036 s   → noise

  qsv frequency  (default, cache short-circuit)
    baseline 1.880 ± 0.028 s
    coldpath 1.864 ± 0.015 s   → noise (paths not exercised)

Outputs identical between builds. The 6% gain is concentrated on the trim
+ ignore-case path because that hot body (lowercase + extend_from_slice +
add_borrowed) is the largest of the closure variants, so isolating its
icache layout has the most leverage.

MSRV 1.95 ≥ cold_path stabilization (1.92).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jqnatividad

src/cmd/frequency.rs

@@ -240,6 +240,7 @@ Common options:
                            CSV into memory using CONSERVATIVE heuristics.
 "#;
 
+use core::hint::cold_path;
 use std::{fs, io, str::FromStr, sync::OnceLock};
 
 use crossbeam_channel;
@@ -2711,6 +2712,7 @@ impl Args {
                             field_buffer.extend_from_slice(string_buf.as_bytes());
                             weighted_add(map, field_buffer, weight);
                         } else {
+                            cold_path();
                             weighted_add(map, field, weight);
                         }
                     }
@@ -2723,6 +2725,7 @@ impl Args {
                             field_buffer.extend_from_slice(string_buf.as_bytes());
                             weighted_add(map, field_buffer, weight);
                         } else {
+                            cold_path();
                             weighted_add(map, trim_bs_whitespace(field), weight);
                         }
                     }
@@ -2858,6 +2861,7 @@ impl Args {
                             field_buffer.extend_from_slice(string_buf.as_bytes());
                             ftab.add_borrowed(field_buffer);
                         } else {
+                            cold_path();
                             ftab.add_borrowed(field);
                         }
                     }
@@ -2870,6 +2874,7 @@ impl Args {
                             field_buffer.extend_from_slice(string_buf.as_bytes());
                             ftab.add_borrowed(field_buffer);
                         } else {
+                            cold_path();
                             ftab.add_borrowed(trim_bs_whitespace(field));
                         }
                     }