document missing-value behavior on every method, add contract test

Every public method's docblock now ends with a one-line "Missing values (null or ...)" contract, so what happens to null and "" is always the last thing you read before the tags:

- string manipulation and raw accessors: pass through / returned unchanged
- math and date formatting: return null (non-numeric input does too)
- conditional methods: skip the operation or apply the fallback
- encoders: return "" (jsonEncode returns JSON literals instead, never empty output)
- type conversions: cast to 0, 0.0, false, or ""

Also in this pass:

- class docblock summarizes the four rules as bullets, one per method group
- examples moved above the contract line wherever they trailed it
- converted the last two @example tags (numberFormat, percent) to indented code examples, one docblock format everywhere
- add MissingValueContractTest: one table asserting every transformation against null and "" input, so the contract can't drift silently

Author: daveedis