Call it 0.1.1 and write down what changed

[thc1006]

From the second round of review on #130: this line changes the submission protocol and the package still called itself 0.1.0.

v0.1.0 writes a pickle, develop writes JSON, and both declared 0.1.0. A competitor holding the package could not tell which one they had, and the leaderboard's /api/version reports what the server accepts without being able to say anything about the client.

The protocol version is the field in the payload, and it did move: format_version 0 to 1. This is the package catching up so the two can be named together. 0.1.1 rather than 0.2.0 because the break is carried explicitly by that field and by the suffix, both of which the leaderboard reports and enforces.

The [Unreleased] section was empty for all 61 commits, in a file that says at the top that it tracks the submission format, scoring and environment behaviour. It is written out now, grouped the way v0.1.0's section is, with the numbers that were measured rather than adjectives: no pop decision moves at the 1.5 m radius over 200,000 segment pairs, and both shipped scenarios divide exactly so the release-spacing change moves nothing.

Tests

Three, in tests/test_release_metadata.py. The drift happened once already and nothing looked, so:

• the declared version has a section in the changelog
• that section says more than a heading
• it has a comparison link

Bump the version without writing the section and all three fail. Checked.

Based on #133, so it carries #132 and #128 as well. Local CI is green: ruff over BalloonPoppingGymEnv/ tests/ doc/examples/ scripts/, uv lock --check, 523 passed and 22 skipped.