fix: put blank lines around changelog headings on the card

[stanlyzoolo]

Problem

GoReleaser writes release bodies with no blank lines at all — ## Changelog / ### New features / ### Bug fixes sit flush against their bullet lists. markdownToLines only ever collapsed blank runs and never inserted its own, so on the card the whole changelog rendered as one dense block (keepkit's own release card is the screenshot case).

Fix

The heading branch in markdownToLines now calls the existing emitBlank() before and after emitting a heading. emitBlank is idempotent (adds a blank only when the previous line is non-empty, drops leading ones), so:

• packed GoReleaser bodies get one blank row on each side of every heading;
• bodies whose author already wrote the blanks come out byte-identical — the inserted blank and the source blank collapse into one;
• a heading that converts to nothing (badge-only) still yields exactly one blank, via the collapse inside mdEmitInline;
• no leading blank before the first heading, trailing blanks trimmed as before.

Tests

• headings h1 to h6 and TestMarkdownToLinesGitHubBody expectations updated for the new spacing.
• Two new table cases: packed headings get air on both sides (the GoReleaser shape) and authored blanks around headings do not double — both pin identical output for the packed and pre-formatted variants.
• Mutation-checked: reverting the two emitBlank calls turns all three assertions red.
• Card visually verified at 120 and 80 columns.

Preflight green: build / vet / test -race / golangci-lint.