fix(isMACAddress): preserve caller EUI options

[FanWu-ai]

isMACAddress('01:23:45:67:89:ab', Object.freeze({ eui: 48 })) throws instead of returning a boolean. The function writes its normalized EUI value back into the caller's options, even when the supplied value is already a string. It also changes a mutable options object's numeric eui to a string.

Normalize EUI into a local variable and use it for the existing comparisons. Validation rules remain unchanged. Regression coverage checks frozen numeric/string EUI-48 and EUI-64 options, the no_separators and legacy no_colons options, valid/invalid addresses and preservation of a mutable options object.

Validation on Linux / Node 24.19.0:

• Full unchanged npm test: 323 passing.
• Full tests with regressions and original source: 323 passing, 2 failing.
• Final npm test: 325 passing, including all builds, minification, ESLint and coverage.
• 6,300 additional assertions across six source/generated entry points confirm frozen-option results match mutable-option results.
• git diff --check passes.

Statement, function and line coverage remain 100%. Branch coverage changes from 96.59% (1697/1757) to 96.57% (1689/1749), because eight covered branches disappear when repeated optional chaining is replaced by the local variable; the number of uncovered branches stays 60. The remote Node-version CI matrix has not been run locally.

AI assistance was used to prepare the implementation and regression tests.

Checklist

• Only related source and regression-test changes
• README reviewed; existing API and validation options are unchanged
• Regression tests added
• References: reproduction uses the existing documented options