fix: omit Vary for static origin

[vibhor-aggr]

Summary

Closes #317.

Do not add Vary: Origin when origin is configured as a static string. Keep Vary: Origin for reflected/dynamic origins, including origin callbacks that return a request-specific origin.

Changes

• Stop appending Vary: Origin for explicit static string origins.
• Preserve Vary: Origin for origin callback results and existing reflected-origin paths.
• Update README header comments and configuration notes for the static-vs-reflected behavior.
• Add regression coverage for static origins and origin callback results.

Verification

PATH=/mnt/TBT_DATA/nipun/work/tasks/earn-100-legal/workspace/toolchains/node/node-v24.16.0-linux-x64/bin:$PATH npm test
PATH=/mnt/TBT_DATA/nipun/work/tasks/earn-100-legal/workspace/toolchains/node/node-v24.16.0-linux-x64/bin:$PATH ./node_modules/.bin/mocha --require test/support/env --grep 'static origins|origin callback returns a specific origin|regexp|array of origin checks'
git diff --check

Results: full suite passed with 50 passing tests and 100% coverage; focused suite passed with 5 passing tests.