Release 0.3.0

[jwinarske]

Prepares 0.3.0 and corrects two packaging problems found while checking
release readiness.

LICENSE did not reproduce the license it named

The file was headed "Apache License 2.0" but three sentences of the operative
text had been reworded and the appendix was absent:

Line	Apache-2.0	Was
51	submitted to Licensor	submitted to the Licensor
63	received by Licensor	received by the Licensor
109	excluding those notices	excluding any notices

A modified Apache-2.0 is not Apache-2.0, and license scanners could not
identify it — pana scored 0/10 on "Use an OSI-approved license". Replaced
with the verbatim 201-line text, checked against two independent canonical
copies which agree on the whole operative section.

Version

0.2.0 to 0.3.0. Two breaking changes, matching the convention used for 0.2.0
which labelled a wire-format change breaking:

• changedMask added to GATT characteristic props. Requires a matching native
  library rebuild.
• startNotify() and stopNotify() now throw where they previously always
  succeeded, when the Notifying property does not reach the requested state.

The changelog entry covers the notification-state fixes merged in #7 along
with the packaging changes here.

Packaging

• hooks constraint raised to ^2.1.0. Verified rather than assumed: with
  .dart_tool/hooks_runner removed, the build hook rebuilds libbluez_nc.so
  from scratch under 2.x, analyzer is clean, and all tests pass.
• Description shortened from 203 to 136 characters. Search engines display
  only the first part, and pana wants 60 to 180.
• Dropped the documentation: URL, which pointed at a pub.dev page that does
  not exist. pub.dev generates documentation once a package is published.
• Added example/README.md describing each example and how to run it.

Verification

pana	160/160, up from 120/160
dart pub publish --dry-run	0 warnings
C++ build / ctest	0 errors / 42 of 42
clang-tidy (scripts/clang_tidy.sh)	passed
dart analyze / dart test	clean / 16 of 16

All gates re-run after the hooks bump rather than before it.

Note on tagging

publish.yml triggers on v*.*.* and runs dart pub publish --force, but
bluez_native is not yet on pub.dev. Automated publishing is configured per
package on its pub.dev admin page, which requires the package to already
exist, so the first publish has to be done manually. Tagging before that is
likely to fail the publish job.