perf(assert): reduce lowercase forks in assert_contains_ignore_case

[Chemaclass]

Problem

assert_contains_ignore_case (src/assert.sh, ~lines 293-320) lowercases both operands with a printf | tr pipe each — 2 subshell forks per call:

expected_lower=$(printf '%s' "$expected" | tr '[:upper:]' '[:lower:]')
actual_lower=$(printf '%s' "$actual"   | tr '[:upper:]' '[:lower:]')

Lower priority than #844 (this assertion is less common), but same class of avoidable per-call fork.

Bash 3.0+ constraint (why the obvious fixes are out)

• ${var,,} — Bash 4.0+. Forbidden.
• shopt -s nocasematch — Bash 3.1+. Forbidden (hard floor is 3.0; CI runs a 3.0 job).
• So tr stays the portable lowercasing tool.

Possible fixes (pick the one that measures best and stays 3.0-safe)

1. Collapse two forks into one — lowercase both operands in a single tr pass over a delimited string, then split. Saves one fork per call.
2. Herestring instead of pipe — tr ... <<< "$s" avoids the printf subshell (one fork instead of two per operand). Confirm <<< behavior on Bash 3.0.
3. Leave as-is if neither is a clean, provable win — acceptable outcome; document why.

Do NOT regress correctness for a micro-fork. Bench first.

Acceptance criteria

• Fork count per call reduced, or a documented decision to keep as-is.
• assert_contains_ignore_case behavior unchanged (existing tests green; add one if the lowercase path is under-covered).
• ./bashunit tests/ + ./bashunit --parallel --simple --strict tests/ green.
• make sa, make lint, ./build.sh --verify green.
• Before/after micro-benchmark in the PR.
• Bash 3.0+ compatible (no ${var,,}, no nocasematch, no printf -v).

[Chemaclass]

Investigated, closing as not a clean win (per this issue's own escape hatch).

Benchmarked the candidate fixes on Bash 3.2 (macOS), 20 000 iterations:

printf '%s' "$x" | tr   : 43.25s   (current)
tr '[:...]' '[:...]' <<< : 45.43s   (herestring — slightly SLOWER)

• Herestring is marginally slower — <<< materializes a temp file; the $()+tr fork dominates either way, and the printf in the pipe is a cheap builtin.
• Combining both operands into one tr is unsafe: the only in-band split delimiter (newline) can appear in a multiline actual, and NUL can't live in a Bash variable.
• Pure-bash lowercase is slower for non-trivial/multiline strings (.claude/rules/perf-fork-budget.md documents tr as the accepted case-conversion path).

assert_contains_ignore_case is also low-frequency, so there's no hot-path pressure. Keeping the current printf | tr form. The high-value sibling #844 (per-assertion join fork) is fixed in #846.