feat(uri): make Authority/PathAndQuery::from_static const

[WaterWhisperer]

Changes

• Authority::from_static and PathAndQuery::from_static are now const.
• Both perform compile-time validation equivalent to their runtime counterparts using byte-wise scanning.
• For const panics on invalid input, uses an out-of-bounds indexing trick compatible with MSRV 1.49 .

Explanation

Uri::from_static remains non-const. Two blockers under MSRV 1.49:

1. Const slicing of &'static str is unavailable, but Uri parsing requires slicing the input into scheme/authority/path+query.
2. Non-standard schemes use Box, which requires allocation not allowed in const fn.

Tests

• cargo test passed.
• Doc tests for const usage included implicitly via examples.

Related issue

Partially fixes #750

[WaterWhisperer]

Hello @seanmonstar

This PR has been open for some time, and I'd like to politely request a review (mainly marking Authority/PathAndQuery::from_static as const, so it can be used in const contexts). Thanks!