fix: clamp current_age to >=0 for future Date headers (RFC 9111 §4.2.3)

[andig]

Closes #2

Problem

When a response carries a Date header ahead of the local clock (clock skew), the freshness logic computes a negative current age. A response with no freshness information (no Cache-Control, no Expires) has a freshness lifetime of 0, so the test lifetime > currentAge becomes 0 > -2s → true, and the response is wrongly marked fresh and served from cache.

RFC 9111 §4.2.3

The age algorithm clamps the apparent age:

apparent_age = max(0, response_time - date_value)

so current_age can never be negative. The max(0, ...) was missing from the three freshness sites (getFreshness, isActuallyStale, checkStaleIfErrorLifetime), all of which used clock.since(date) directly.

Change

Add a small clampedAge(date) helper (max(0, now - date)) and use it at the three sites. With the clamp, a future-dated response with no freshness info evaluates 0 > 0 → false → correctly stale.

A regression test (TestFreshnessFutureDateClockSkew) is added; it fails before the change and passes after. Full test suite remains green.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes HTTP freshness evaluation when a cached response’s Date header is ahead of the local clock (clock skew), ensuring current_age never becomes negative per RFC 9111 §4.2.3 so future-dated responses aren’t incorrectly treated as fresh.

Changes:

• Introduce clampedAge(date) to clamp now - date to >= 0 per RFC 9111 §4.2.3.
• Use clampedAge in getFreshness, isActuallyStale, and checkStaleIfErrorLifetime to prevent negative age affecting freshness decisions.
• Add a regression test covering the future-Date clock-skew scenario.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
httpcache.go	Adds clampedAge helper and applies it to freshness/staleness calculations to prevent negative current_age.
httpcache_clockskew_test.go	Adds regression test ensuring a future Date header with no freshness info is treated as stale.

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