Fix #198 — %%stretchlast and %%stretchstaff, as abcm2ps implements them

[sbeitzel]

Fix #198

The last line of a tune was drawn at natural spacing, which packs eighths almost head to head next to the justified lines above it. It now behaves as abcm2ps 8.14.0 does (measured against its output):

• %%stretchlast F (0…1, default 0.25): the last system is stretched to the line when its unstretched staff (header + music) reaches 1 − F of it. The spec's logical form is accepted as 1/0; values outside 0…1 are rejected with an invalidStretchLast warning, as abcm2ps rejects them.
• A short last system takes the previous line's spacing, never below natural and never past the line. This is what fixes the issue's repro, whose last line is only 19% full.
• %%stretchstaff false stretches no system at all. ckprobe --natural now forces it, so it reports natural widths for every system (with the rule above, %%stretchlast 0 no longer gives a natural last line).

API changes (need an ## Upgrading entry)

• %%ceolkit:justifylast true|false is deprecated: parsed as %%stretchlast 1|0 with a deprecatedDirective warning.
• CeolKitDirective.justifyLast(Bool) → .stretchLast(Double) and .stretchStaff(Bool).
• SVGRenderConfig.justifyLastSystem → stretchLast (default 0.25) and stretchStaff (default true).
• Justifier.justify/justifyGroups take stretchLast: (and optional stretchStaff:) in place of justifyLastSystem:.
• The last line of most tunes changes.

Tests

• Justifier: stretch threshold, previous-line spacing, stretchStaff: false.
• Parser: %%stretchlast numbers/logicals/invalid values, %%stretchstaff, deprecated %%ceolkit:justifylast.
• Re-recorded the Canzonetta and Zocharti Loch page snapshots (last systems now stretched or spaced like the line above). One style test was split because its fixture's full last line is now stretched by default.

Docs: EXTENSIONS.md, README.md, CLAUDE.md.

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