Resolve all clippy and rustc lint warnings across the feature powerset

[asmello]

Resolves all clippy and rustc lint warnings across the full feature powerset, prep for the next release. No public API changes — internal/tooling only, so no version bump here.

What was flagged & fixed

clippy::all / clippy::pedantic errors (the crate denies both at lib.rs:38):

• explicit_auto_deref in StringOrToken::read_span (&**self → self).
• Redundant .into_iter() before .collect() and String::from("") → String::new() in tests.
• Bare URLs in README.md (included as crate docs via #![doc = include_str!]) wrapped in <…>.

Newer-toolchain rustc lint mismatched_lifetime_syntaxes (9 sites): made elided output lifetimes explicit in pointer.rs public signatures — Tokens<'_>, Token<'_>, Components<'_>. Purely syntactic; the API is unchanged.

#[macro_use] extern crate alloc: removed. It was unused in the minimal feature set (warning under --no-default-features). vec!/format! are now imported where used — inline use for single-use sites, matching the codebase's existing style.

Feature-combination dead code: gated items so they don't warn when their consumer features are off — Assigned (json/toml), and parse_index + its Token import (json).

Verification

• cargo hack --feature-powerset --no-dev-deps clippy → 0 warnings
• cargo clippy --all-features --all-targets → clean
• cargo test --all-features → 104 + 48 pass; cargo fmt --check clean

One note: clippy --fix tried to reduce Some(ptr).unwrap_or_default() to ptr in the default_lifetime_is_correct regression test (#111), which would remove what the test checks. That was reverted and given a local #[allow(clippy::unnecessary_literal_unwrap)] instead.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.6%. Comparing base (c96d8dd) to head (6a64eb0).

Files with missing lines	Patch %	Lines
src/index.rs	83.3%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/assign.rs	97.4% <ø> (ø)
src/pointer.rs	95.8% <100.0%> (ø)
src/resolve.rs	92.4% <ø> (ø)
src/token.rs	98.3% <100.0%> (-0.1%)	⬇️
src/index.rs	90.6% <83.3%> (-0.3%)	⬇️

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