Reorganize examples into three tiers; promote ListLayout into the library

[PeteRichardson]

Reorganizes the package's ad-hoc examples into three tiers, and promotes list-layout
logic that was duplicated across two examples into the library.

Implements the design spec
via this plan (both included in this PR).

What changed

Tier 1 — tutorial snippets. New top-level Snippets/ directory, auto-discovered
by SwiftPM with no Package.swift entry. Five ~15-line tutorials, each teaching one
thing, compiled on every build so a tutorial referencing a renamed API breaks CI
instead of quietly misleading a reader.

Tier 2 — labelprint. Replaces example-listlabel. Reads lines on stdin and
prints them as one long label; keeps only stdin parsing now that layout lives in the
library. Subcommands print (default), preview, zpl, list. Drops the
example- prefix, since these are meant to be symlinked into ~/bin.

Tier 3. compare-renderers renders the same ZPL through both Labelary and
zpl2png to expose renderer disagreements — this is the ZPL-debugging tool.
batch-badges renders one Stencil template over a CSV to make N labels. LabelGUI is
now documented as the Tier 3 GUI example.

Library: ListLayout. generate_label was duplicated across two examples and
had never been tested. It is now one public ListLayout type in
Sources/LabelKit/Labels/, with 10 unit tests. This is what unblocks
t#42 — t can consume it rather
than copying it a third time.

Library: StencilTemplateStore.init(fileURL:). The store previously resolved
only to Application Support / group-container paths, so examples depended on
invisible machine state. example-label ran locally only because LabelGUI had left
a templates.json in a group container; on a fresh clone it threw. Examples now
read seed templates checked into the repo.

CI. The repo had none. This matters beyond hygiene: snippets' entire anti-rot
guarantee is that swift build compiles them, which is only true if something runs it.

Docs. The DocC catalog referenced StencilProcessor and LengthInjector —
neither has ever existed. Corrected to ResolveTemplates / InjectLength.

Notable fixes found during review

• CRLF input was completely broken in the list parser. Swift treats \r\n as a
  single Character, so split(separator: "\n") never split CRLF text — a 5-line
  list parsed to one item with raw CR bytes landing inside ^FD. Any
  Windows-authored or pasted list hit this. Fixed with tests.
• ListLayout.printWidthDots ignored its environment, hardcoding 1200. It
  coincidentally matched 4-inch stock, which is why nothing caught it; on label2x1
  the ZPL declared 1200 dots while the geometry said 600, so previews and print
  would disagree. Now derived from the environment, with an explicit override.
• compare-renderers aborted before the offline renderer if the network one failed,
  forfeiting its entire offline value. Each renderer is now isolated, and each
  reports its PNG dimensions — which is how you see the disagreement.

Deliberately not done

• example-reminderlist survives. It was to move to a new reminders repo that
  does not exist yet; deleting the only copy of Reminders.swift would be
  destructive. Docs still describe it. Removing it is follow-up work.
• jazzy → DocC migration (deferred by the spec; it is what would make //! and
  @Snippet render).
• ZPL improvements Emit leaner, correcter ZPL: ^CF default font, ^CI28 UTF-8, ^FH escaping #53 and Use ^FB field blocks for list layout instead of host-computed per-line coordinates #54 — now easier, since ^FB work lands in one library
  place next to the length estimator instead of in two examples.

Verification

swift build clean; 110 tests passing (up from 89). Every example was exercised
end-to-end. Nothing was sent to the physical printer at any point.

[PeteRichardson]

lgtm