Enforce two conventions in lint that nothing was checking

[dyve]

Two rules this repository has stated for a long time and never enforced, so both held only while someone remembered. Each is a git grep or a grep in just lint, in the same shape as the existing _no-invisible-characters recipe.

Trailing whitespace

.editorconfig has set trim_trailing_whitespace = true for every file since the beginning, and nothing checked it. ruff only sees Python, and the invisible-character check looks for a specific set of code points that does not include a plain space. Two lines had slipped past: django-bootstrap4's .github/dependabot.yml and django-icons' docs/index.rst.

lint checks, format fixes, matching the existing contract. Both are scoped by git grep -lIP, so -I excludes binaries and the pattern excludes files already correct — on a clean tree the fixer is never invoked.

Markdown hard line breaks are preserved. Two trailing spaces in Markdown are a hard break, not an accident, and an earlier revision of this branch deleted them silently. format now passes --markdown-linebreak-ext=md, so exactly two spaces survive while one or three are still stripped, and the check excludes .md. The two have to agree: a check that failed on something format deliberately preserves would leave you stuck between them.

Checking rather than auto-fixing elsewhere is deliberate. These packages render HTML, and trailing space inside a template is output, so stripping it can change a rendered page. That belongs in a diff a person reads.

On the "is there a standard" question: yes, and this uses it. format runs trailing-whitespace-fixer from pre-commit-hooks through uvx, the same implementation the pre-commit framework runs for this hook, invoked the way this justfile already runs ruff, typos and twine. No .pre-commit-config.yaml, no pre-commit install step for contributors, no second place where linting is configured.

One ## Unreleased heading

Branches that each add the heading instead of appending to the existing one merge without a conflict, because the two sides touch different lines. This repository reached four before anyone looked, and several were mine. The consequence is not cosmetic: cutting a release stamps the first heading and leaves the rest, so those entries ship as pending notes for a version that already went out. That is the failure #694 was written to prevent, arriving through a door the changelog itself left open.

The check also rejects the heading sitting below a released section, and deliberately allows none at all, which is the correct state between a cut and the next change.

This branch's rebase consolidated the four headings into one, so main is already repaired by merging it.

Verified

Clean tree passes and format touches nothing. Trailing whitespace outside Markdown fails lint and is fixed by format. A Markdown hard break survives both. A single stray space in Markdown is still cleaned. Two ## Unreleased headings fail with the line numbers printed, a misplaced heading fails, and no heading passes.

Propagation waits on django-bootstrap4#912 and django-icons#668, since the whitespace check would fail in those two until their stray lines are gone.

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