Add PresentationDocument and export presentation subcommand (#38, #39)

[leogdion]

Closes #38. Closes #39.

Rebuilt on v0.1.x now that #42, #43, and #44 have landed. This branch previously merged all three so it could build; the diff is now just the presentation work — 9 files, one commit.

What this adds

The demo deck: seven solid-color slides with styled titles anchored on lane 1, separated by one-second cross dissolves. No .mov files and no ffmpeg — anyone who clones the repo can regenerate the .fcpxml with one command. FCPKit demonstrating itself.

PresentationDocument and PresentationSlide ship in FCPKitDSL so the showcase is browsable and testable; the CLI holds only a thin command that invokes them, per the RGBDocument lesson.

PresentationSequence assembles story items from an array, because a deck's length is data-driven rather than fixed-arity like the result builder's buildBlock.

Dissolve-safe title timing

Anchored items are not swept into a primary-storyline transition, so a title spanning a dissolve would hard-cut while its background dissolved. Packing sets a dissolved clip's start to T/2 and shrinks its duration by both overlaps, and an anchor's offset is relative to that trimmed start — so a title at offset zero already begins where the incoming dissolve ends, and only the tail needs trimming:

titleDuration = slideDuration - (incomingT / 2) - (outgoingT / 2)

Asserted directly rather than assumed.

Cleanup

Deletes the duplicated library copy of RGBDocument. Demo scaffolding belongs only in the CLI, and FCPTimeIntervalTests already used its own local fixture, so nothing broke.

Verified end to end against Apple's shipped DTDs

$ swift run fcpxml-dsl export presentation presentation.fcpxml
$ xmllint --noout --dtdvalid FCPXMLv1_14.dtd presentation.fcpxml   # ✅ valid
$ xmllint --noout --dtdvalid FCPXMLv1_13.dtd pres113.fcpxml        # ✅ valid

• Packs to 36s, inside the 30–45s target (asserted on the packer's computed duration, not hand arithmetic)
• Emits ts1..ts7 — seven unique text-style-def ids (Fix duplicate text-style-def ids in multi-title documents #35 working in the showcase)
• The 1.13 export carries zero match-analysis-type elements (DSL emits match-analysis-type at v1.13, where the element does not exist #41 working)
• Effect interning: exactly one Custom, one Basic Title, one Cross Dissolve across seven slides
• Byte-identical to the export produced before the three tracks were squashed

Gate

• FCPKIT_REQUIRE_DTD=1 swift test — 68 XCTest + 49/12/65 Swift Testing, all passing
• fcpxml-diff schema-completeness --fail-if-total-exceeds 0 — no structural loss
• swift-format lint and swiftlint — 0 violations across 282 files
• swift run fcpxml-dsl --help lists presentation in USAGE and EXAMPLES
• Neither rgb.fcpxml nor presentation.fcpxml appears in git status after an export run

Remaining

#40 is human-only — it needs Final Cut Pro for the Share → Master File export and the README embed.

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