Docs: README and repo hygiene (2026-09-25 audit)

[pietechsolution]

Held for review. Please do not merge yet.

Changes

• README first paragraph now says what the package is (Flutter seat map SDK), followed by a short company line. Every README em dash rewritten.
• design/picker-spec.md: internal decision notes removed and each rule stated neutrally. "What it unlocks" -> "What it enables".
• lib/ and test/: internal decision comments rewritten neutrally with the same technical meaning. One test description string is shortened (an internal date suffix is removed). Comments and names only, no behaviour change.
• The version is not bumped.

Checks

• scripts/check-public-repository.sh passed.
• dart format on the changed files matches the formatter (one test call reflowed after its name got shorter).
• flutter test on the 8 changed test files: 129 passed, 28 golden failures. The same 28 goldens fail on an unmodified checkout on this macOS machine (the goldens are Linux-rendered), so this change adds no failures.