fix(parser): lower the next-cast copy in the possessive word order (CR 603.7)

[matthewevans]

Twinferno's "When you CAST YOUR NEXT instant or sorcery spell this turn, copy
that spell." produced no delayed trigger at all — the copy was dropped outright,
and its "You may choose new targets for the copy" sentence was left as an honest
orphaned_copy_retarget residual. The orphan was the symptom, not the disease:
the retarget clause was correctly refusing to bind to a copy that did not exist.

try_parse_when_next_event dispatched on alt((tag("when you next cast a "), tag("when you next cast an "))). Magic prints this one clause in two word orders
and we only accepted the adverbial one:

"when you NEXT CAST a[n] this turn" Galvanic Iteration, Doublecast
"when you CAST YOUR NEXT this turn" Twinferno

Add the possessive opening to that alt. The payload after either opening is the
same "[type-phrase] spell [post-modifier]" shape, so extract_when_next_spell_filter
and every downstream stage are untouched — this parameterizes the opening rather
than proliferating a second parser for the same clause.

Filed under #62, whose premise this partly FALSIFIES: the task described Twinferno
as a MODAL-nesting gap ("copy not lowered inside a modal delayed trigger"). It is
not. Modes are irrelevant — the identical text outside a mode fails the same way,
and the identical text in the adverbial word order inside a mode works. The gap is
purely the opening's word order.

Honest scope, measured rather than assumed: "cast your next" appears on 3 cards in
the pool, but Flaming Fist Duskguard and Valiant Farewell print it inside a quoted
boon with no " this turn, " delimiter, so they stay gapped and this fix does not
claim them. The measured delta is 1 face.

Full-pool dual export (35,396 faces), whole-face structural diff — every face's
ENTIRE tree compared, not just the copy-retarget channel:

faces with ANY structural change: 1
twinferno retargets=[] orphans=1 -> retargets=[MayChooseNewTargets] orphans=0

Zero regressions. Positive controls Galvanic Iteration and Doublecast unchanged.
Pool-wide orphaned_copy_retarget residuals: 7 -> 5.

[gemini-code-assist]

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[github-actions]

Parse changes introduced by this PR · 1 card(s), 2 signature(s) (baseline: main 9aad1a5deaca)

1 card(s) · ability/CreateDelayedTrigger · added: CreateDelayedTrigger (when=when next event this turn)

Examples: Twinferno

1 card(s) · ability/when · removed: when

Examples: Twinferno

1 card(s) had Oracle-text changes (errata/reprint) — excluded as non-parser.