Optional per-level heading styles (H1–H5)

[joestump-agent]

Part of #54. Depends on the token-set expansion (#56).

Problem

renderText (render/text.go:10-17) collapses heading levels: h1/h2/h3 all render as Heading, h4/h5 as Subheading. A themed host may want per-level distinction (size is not available in a terminal, but color/underline/case treatment are).

Fix

• Add optional H1, H2, H3, H4, H5 lipgloss.Style overrides to Styles.
• Resolution rule: a zero-value H<n> falls back to Heading (h1–h3) / Subheading (h4–h5), so DefaultStyles() output is unchanged and existing hosts that only set Heading/Subheading keep working.
• Decide and document zero-value detection (lipgloss styles aren't trivially comparable — either track set-ness with a small helper or compare against lipgloss.NewStyle() rendering; pick one and pin it with a test).
• Test: default output unchanged; with H1 overridden, only h1 text changes.

Low priority relative to #56/#57 — file under "nice for hosts that want editorial hierarchy".