Fix Token::from_encoded accepting a double tilde

[asmello]

Summary

• Token::from_encoded set escaped = true unconditionally when it saw ~, so a second ~ right after an already-open escape (e.g. the second ~ in "~~0") was treated as re-opening the escape rather than being rejected as an invalid escape sequence.
• This let safe code (Token::from_encoded → PointerBuf::from_tokens) construct a PointerBuf whose string form Pointer::parse then refused on the exact same bytes.
• Fix: only transition into the escaped state when not already escaped; a ~ seen while already escaped now falls through to the existing "invalid character after tilde" error path.

Fixes #128

Test plan

• Added a regression test in src/token.rs asserting Token::from_encoded("~~0") returns EncodingError { offset: 1, source: InvalidEncoding::Tilde }
• cargo test --all-features (105 lib tests + 49 doc tests) passes
• cargo clippy --all-features --all-targets is clean

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.6%. Comparing base (3dc1604) to head (d7d933e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/token.rs	98.4% <100.0%> (+<0.1%)	⬆️

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