fix: normalize project names from PEP 621 metadata

[Solaris-star]

Normalize PEP 621 project.name values before merging them into Briefcase configuration so generated application identifiers remain valid. Legacy Briefcase configuration using an invalid project name now emits a FutureWarning, preserving compatibility while announcing the future validation behavior.

The change includes focused regression tests and a Towncrier change note.

Fixes #3011

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by: Hermes Agent

[freakboy3742]

As noted on previous PRs you've submitted - there's no need to include a "testing" regimen in the PR description. The required testing regimen is "it passes CI" - which, at time of review, this PR does not.

The contribution guide for details on how to set up a testing environment locally and validate that a PR will pass CI before pushing the PR for review.

[freakboy3742]

1. Please don't force push. We don't care about having a "clean" commit history, because we use merge commits when a PR is accepted.
2. Please configure your local development environment as described in the contribution guide, and run tests locally before pushing updates. The CI failures that are currently being shown are all locally reproducible if you follow the contribution guide.