Bring back the Duration helpers as disfavored overloads

[Jeehut]

Keeps the next ErrorKit release source-compatible with 1.2.1.

1.2.1 ships Duration.minutes(_:), Duration.hours(_:) and Duration.timeInterval as public API, and the README and DocC examples call the log APIs with them (ErrorKit.logAttachment(ofLast: .minutes(30))). On main, c9c2fe8 made timeInterval internal and removed the other two, because HandySwift defines the same three helpers: in a file that imports both modules – as everything built on FlineDevKit does – .minutes(30) was ambiguous. Releasing main as it is would turn that fix into a compile error for every app that uses ErrorKit without HandySwift, including the README's own sample.

This branch brings the three helpers back as public, marked @_disfavoredOverload. The attribute only lowers their rank when another candidate exists, so:

File imports	.minutes(30) resolves to
ErrorKit only	ErrorKit's helper (as in 1.2.1)
ErrorKit + HandySwift	HandySwift's helper, no ambiguity

Alternatives considered: shipping the removal as 2.0.0 (a major release for three convenience helpers, while the docs keep needing a replacement for .minutes(30)), or removing the helpers from HandySwift instead (moves the break to HandySwift's users).

How this was verified

DurationHelperTests checks the three helpers from a plain import ErrorKit. Against the sources on main it fails to compile; on this branch the full suite passes:

# sources from main
error: type 'Duration' has no member 'minutes'
error: type 'Duration' has no member 'hours'
error: 'timeInterval' is inaccessible due to 'internal' protection level

# this branch
✔ Suite "Duration helpers" passed
✔ Test run with 21 tests in 5 suites passed.

The two-module case was checked with a throwaway package that has one executable importing ErrorKit and HandySwift (local checkouts) and one importing only ErrorKit, both calling .minutes(30), .hours(1).timeInterval and ErrorKit.loggedData(ofLast: .minutes(5)). Both built and ran. Without @_disfavoredOverload, the two-module target failed again:

Sources/Both/main.swift:4:20: error: ambiguous use of 'minutes'