feat!: raise the minimum bash from 3.0 to 3.2

[Chemaclass]

🤔 Background

Breaking change. 3.2 is the bash macOS ships, which is the reason for supporting an
old bash at all. 3.0 (2004) and 3.1 (2005) have no practical install base, and supporting
them has a real cost: they lack printf -v and +=, and predate the 3.2 change to
[[ =~ ]] quoting — so at a 3.0 floor a regex match behaves differently across
"supported" versions, which is worse than not supporting them.

The gate also never enforced the declared minimum: it compared only the major, so every
3.x passed.

💡 Changes

• Gate on major and minor; 3.0/3.1 are refused with a clear message. Verified against a real Bash 3.0 build, which is now correctly rejected
• CI builds and tests a real Bash 3.2 instead of 3.0; the unit suite passes on it (1105 tests)
• Update README, docs and install.sh to state 3.2+

🔧 Two rules corrections this work forced

• perf-fork-budget.md claimed "Bash 3.0 has no [[ =~ ]]; the grep -E fork is mandatory". [[ =~ ]] works on a real 3.0 build. That fork follows from the house style rule preferring [ ], not from the version floor — so it is removable if the style rule is revisited
• bash-style.md listed [[ ]] alongside genuinely 4.0+ constructs. Version constraints and style preferences are now separated, so the latter is not mistaken for the former

⚠️ Method note

Docker validation must invoke the script as an argument to the old bash
(/opt/bash-3.2/bin/bash ./bashunit …), as CI does. Using bash -c './bashunit …' goes
through the #!/usr/bin/env bash shebang and silently runs the container's default bash
5.x instead — I hit exactly that and re-verified everything.