Write cookie attributes one value per call in stringify

[quinnj]

One small change so an HTTP.jl server compiled with juliac --trim=safe verifies cleanly, found while trimming a Servo application on Julia nightly (JuliaCon 2026 workshop prep).

Base's varargs write(io, x1, xs...) iterates a heterogeneous tuple and dispatches each element dynamically, which the trim verifier cannot resolve (a 5-line program writing write(io, "a", '=', "b") fails verification on nightly). Cookie stringify used it in four places; splitting them into single-value writes produces byte-identical output and compiles to direct calls.

The companion change on the server request path (rebuilds through the positional constructor) already landed in #1349 — an earlier draft of this PR duplicated it and has been trimmed down. Together with #1349, HTTP contributes zero verify errors to that app's --trim=safe build.

Tests: http_cookie_tests.jl passes locally (rerun on this rebased branch), and the core/h1 wire suites passed with the same cookie change before the rebase.

🤖 Generated with Claude Code

Review follow-up

• Add exact request and response cookie rendering coverage.
• Add a trim workload that builds and runs the public cookie stringify path.

Validation: focused cookie tests pass. The new trim executable builds and runs with zero verifier errors.

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.55%. Comparing base (444b92b) to head (70c46a2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1350      +/-   ##
==========================================
+ Coverage   88.53%   88.55%   +0.02%     
==========================================
  Files          31       31              
  Lines       11961    11974      +13     
==========================================
+ Hits        10590    10604      +14     
+ Misses       1371     1370       -1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.