fix(multitude): restore coverage for transcode_utf16_into

[martin-kolinek]

Problem

PR #478 (feat(thread_aware): add feature-gated ThreadAware impls for 3rd-party crate types) dropped codecov coverage on main from 100% to 99.9% even though it didn't touch the multitude crate at all (its own patch coverage was 100%).

The 11 missed lines are all the body of the private transcode_utf16_into helper in crates/multitude/src/arena/alloc_utf16.rs: lines 213, 219–225, 227, 228, 230.

Root cause

The helper is #[inline(always)], so no out-of-line function body exists in the compiled test binary. cargo llvm-cov has to attribute hits to the inlined copies via debuginfo, and that mapping is fragile — it shifted when PR #478 added new optional deps (bytes, http, jiff02, uuid) to thread_aware, which is a transitive dep of multitude via bytesbuf. The function is still exercised by existing tests (alloc_utf16_str_arc_from_str, the oversized routes tests added in #487, etc.); coverage just stops getting attributed to the source lines.

Fix

Gate the inline(always) on not(coverage_nightly):

#[cfg_attr(not(coverage_nightly), inline(always))]

Under coverage instrumentation the function keeps a real callable body that llvm-cov attributes hits to deterministically. Release/dev/CI test builds keep inline(always) exactly as today — zero perf change for users.

The coverage_nightly cfg is already a known idiom in this crate (used widely with #[coverage(off)] and feature(coverage_attribute)).

Verification

Ran cargo +nightly-2026-05-30 llvm-cov --all-features -p multitude --lcov locally:

• Before: alloc_utf16.rs reports 72 hit / 11 missed (exactly lines 213, 219–225, 227, 228, 230 — matches what codecov shows on main).
• After: alloc_utf16.rs reports 83 hit / 0 missed. 100% restored.

[Copilot]

Pull request overview

Restores deterministic llvm-cov line attribution for multitude’s private UTF-16 transcoding helper by avoiding forced inlining during coverage builds, addressing a recent project coverage drop without changing runtime behavior for normal builds.

Changes:

• Replaces unconditional #[inline(always)] on transcode_utf16_into with #[cfg_attr(not(coverage_nightly), inline(always))].
• Adds an inline comment explaining why coverage builds should keep an out-of-line function body for stable line mapping.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (fc13464) to head (05f5f2e).

Additional details and impacted files

@@          Coverage Diff           @@
##            main     #499   +/-   ##
======================================
  Coverage   99.9%   100.0%           
======================================
  Files        336      336           
  Lines      24829    24829           
======================================
+ Hits       24818    24829   +11     
+ Misses        11        0   -11     

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