feat: --toc-depth takes a level range like 2-6

[ralsina]

What

--toc-depth now also accepts a range: N keeps its meaning (levels 1..N), while N-M lists only levels N through M, inclusive.

• --toc --toc-depth 2-6 — everything except a level-1 document title: the usual markdown book (H1 = title, H2 = chapters) gets a real TOC by default shape instead of a single self-referential entry
• --toc --toc-depth 2-2 — chapter titles alone
• --toc --toc-depth 2-3 — chapters and their sections, title skipped

Validation is strict: integers 1..6, N ≤ M, and only the forms N or N-M (3-2, 2-7, 0-2, 2-, soon, 2-2-2 all abort with a usage message on stderr). No silent clamping.

Semantics

• The window filters the TOC listing only — PDF bookmarks/outline keep every heading (a top-level title bookmark is conventional and useful), and the mtoc-N anchor numbering stays aligned because the filter lives in toc_html, not in the heading map.
• The two-pass settle loop is untouched: it already rebuilds the TOC block each pass, and a smaller listing just settles like a smaller document.
• warn_missing_toc learned the range vocabulary: "--toc found no headings at depth 2-6" (lone N still reads "at depth N").

Library

Additive kwarg toc_min_level : Int32 = 1 on Markd::Pdf.render and render_to_memory; settled_pages takes it as a defaulted parameter before the block, so existing callers compile unchanged.

Docs

Help text and the README docopt block were updated together and verified byte-identical to the actual --help output; the README TOC section documents the range form with the title-skip idiom.

Verification

• 4 new specs in spec/pdf_toc_spec.cr: window filtering (including the "only level 2" case), nil when everything falls outside the window, a real two-pass render of a title+chapters+sections book at toc_depth: 3, toc_min_level: 2 asserting the title stays out while every listed number matches the heading's true page, and a CLI spec covering 2-6 plus six rejected malformed values
• Full suite: 257 examples, 0 failures (1 pre-existing pending); ameba 0 failures; all four binaries build (fresh worktree: submodules + shim + shards build)
• Smoke-tested: 2-6 and 2-2 renders, --print-config dumping toc_depth: 2-3, config-file roundtrip (toc-depth: 2-4 read back), kdp + range with recto numbering, both warning paths, and the usage errors above