fix(kafka): restore 40 percent automatic heap sizing

[weicao]

Problem

Task #54 formal runtime on addon candidate 67200df711766c01ec7a33829c1b0ee041b1f32a reached the regression R2 check with the live Kafka process using -XX:MaxRAMPercentage=100. The candidate templates and README contain the same 100 percent value, while merged PR #2855 is not in the candidate ancestry.

Change

• Integrate PR chore(kafka): reduce MaxRAMPercentage from 100 to 75 #2824's container-memory input and safer fallback heap defaults.
• Integrate PR chore(kafka): reduce auto heap ratio to 40 percent #2855's 40 percent automatic heap calculation for Kafka 2.7 and current server setup paths.
• Keep this as a draft stacked on PR fix(kafka): block unsafe KRaft controller scale-in #3219's exact candidate branch until the stack is aggregated onto a valid release base.

Verification

• bash -n passes for both server setup scripts.
• Kafka ShellSpec: 30 examples, 0 failures on Bash 5.3.
• Helm dependency build and helm template succeed.
• Rendered chart and Kafka source/examples contain no MaxRAMPercentage=100.
• Direct function check with a 1024 MiB limit produces -Xmx409m -Xms409m in both setup scripts.
• git diff --check passes.

Runtime retest, package, runner, sideload, evidence, and cleanup remain tester-owned.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (67200df) to head (2067c10).

Additional details and impacted files

@@                          Coverage Diff                          @@
##           audrey/kafka-kraft-scalein-failclosed   #3248   +/-   ##
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  Coverage                                   0.00%   0.00%           
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  Files                                         76      76           
  Lines                                       9426    9426           
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  Misses                                      9426    9426           

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