Remove regexes from compiled code

[ericmj]

Regex literals are compiled into beam files at compile time, and the compiled PCRE pattern format is not portable across OTP versions. An archive built on OTP 28.1+ embeds patterns that are reloaded via :re.import/1, which does not exist on OTP <= 28.0, crashing with:

** (UndefinedFunctionError) function :re.import/1 is undefined or private
    (stdlib 7.0) :re.import(...)
    (hex 2.5.0) lib/hex/cooldown.ex:74: Hex.Cooldown.duration_to_seconds/1

(seen in https://github.com/elixir-lang/elixir/actions/runs/28501655392/job/84480157006)

Only two regexes existed in shipped code; both are replaced with plain pattern matching:

• lib/hex/cooldown.ex: ~r/\A(\d+)(d|w|mo)\z/ becomes Integer.parse/1 with a leading-digit guard. Behavior-identical, verified on 18 edge cases including "+5d", "-5d", "1_0d", whitespace, and non-ASCII digits; characterization tests added.
• lib/mix/tasks/hex.build.ex: ~r/\W\.DS_Store$/ becomes Path.basename(path) == ".DS_Store". User-supplied :exclude_patterns regexes still work since they are built in the user's project at runtime and never embedded in Hex's beams. Two intentional improvements: a root-level .DS_Store is now excluded too, and files merely ending in .DS_Store are no longer silently dropped.

A new lint test rejects any ~r/Regex./:re. usage under lib/ so regexes do not get reintroduced into the precompiled archive.