v3.0.0

Release v3.0.0

Modernise the example, add link-capacity utilisation, and tidy the rate
maths.

- migrate the example from the retired cmdparser_cpp to usage_cpp: the
  options are now declared as a usage_t, help text is generated, and the
  numeric interval is validated by the parser. The cmdparser submodule is
  replaced by usage; common is still required by the library itself.
- add interface_t::link_speed() (Mbit/s from /sys/class/net/<if>/speed, 0
  when unavailable) and interface_t::link_percent() (utilisation relative
  to the link's capacity) alongside the existing peak-relative percent(),
  so both metrics are available depending on the use case.
- replace the magic 1.049/8.3886 factors (which netted to ~x8) with an
  honest bytes*8 bit conversion, and use decimal kilo/mega/giga (1000)
  instead of binary (1024) for network rates. Displayed numbers shift
  slightly.

Bug fixes:
- guard interface_t::update() against counter resets/wraps that would
  otherwise underflow into an astronomically large bogus rate
- guard the unguarded front()/back() in clean_up() and pass unsigned char
  to isspace()/isalpha()
- drop the declared-but-undefined bps_t(interface_t&) and default the
  bps_t copy constructor (silences -Wdeprecated-copy)

Bump the common submodule to a revision that builds on current compilers
(GCC 15), wire the example's keep (-k) option so it actually keeps the
unit, and refresh the GitHub Actions (checkout@v5, create/upload actions
@v2, push/pull_request triggers, submodules in the release checkout).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Oskari Rauta <oskari.rauta@gmail.com>