feat(engine): Twinning Staff copy-count replacement

[danielbrock58]

Implements Twinning Staff and the broader "copy an additional time" replacement class, on a clean base off current upstream (twinning-base) so only the fix is in the diff. Opened on the fork for review/approval before any upstream contribution.

What

"If you would copy a spell one or more times, instead copy it that many times plus an additional time. You may choose new targets for the additional copy."

Approach (building block, not a one-off)

• Modeled as a CopySpell replacement carrying a QuantityModification — the same shape as the token/counter doubling family (Doubling Season, Hardened Scales), so it generalizes to "plus N additional times", not just Twinning Staff.
• Parser composes nom combinators along independent axes (count / additional / singular-plural time(s)); no verbatim-phrase matching. Covers plus an additional time and plus N additional times.
• New CopyCountStatus enum (not a bool) guards the per-copy resume so the bonus applies to the copy event once, not per copy.

Rules fidelity (CR annotations verified against MagicCompRules)

• CR 707.10 — copying a spell vs. an ability (Staff affects spell copies only)
• CR 614.1 / 614.1a — replacement watches for an event that would happen; "instead" replacement
• CR 614.5 — a replacement effect doesn't invoke itself repeatedly (the anti-runaway guard)
• CR 616.1 — additive modifications are order-independent (no ordering choice needed)

Fixes

• Runaway copy loop when copying a targeted spell with Twinning Staff (each per-copy retarget pause re-applied the bonus → dozens of copies). Now exactly base + 1.

Tests

7 regression tests (parser base + plural/numbered variants; engine: +1 copy, zero-copy guard, opponent's Staff ignored, ability-copies excluded, targeted-spell no-runaway). Full engine suite: 434 passed, 0 failed.

Commits

• implement copy-count replacement
• stop runaway copy loop on targeted spells
• typed CopyCountStatus, modular plural parse, zero-copy guard
• correct CR annotations to verified rules

[gemini-code-assist]

Code Review

This pull request implements "copy an additional time" replacement effects (such as Twinning Staff) according to CR 707.10, CR 614.1a, and CR 614.5. It introduces a CopyCountStatus enum to prevent runaway copy loops on resumed iterations, integrates the modified copy-count logic into the resolution chain, and adds parser support for these replacement effects alongside comprehensive tests. The reviewer suggested avoiding casting the base count from usize to u32 to prevent potential truncation on 64-bit systems, recommending instead to keep the count as usize and cast the modification values.

[gemini-code-assist]

Casting base (which is a usize) to u32 can cause truncation on 64-bit systems if the value exceeds u32::MAX. It is safer and more idiomatic to keep count as a usize and cast the u32 modification values (value) to usize instead.

Suggested change

	let mut count = base as u32;
	for (_idx, obj, def) in crate::game::functioning_abilities::active_replacements(state) {
	if def.event != ReplacementEvent::CopySpell || obj.controller != controller {
	continue;
	}
	count = match def.quantity_modification {
	Some(QuantityModification::Double) => count.saturating_mul(2),
	Some(QuantityModification::Plus { value }) => count.saturating_add(value),
	Some(QuantityModification::Minus { value }) => count.saturating_sub(value),
	// `Prevent` / unspecified is not a copy-count increase — leave as-is.
	Some(QuantityModification::Prevent) | None => count,
	};
	}
	count as usize
	let mut count = base;
	for (_idx, obj, def) in crate::game::functioning_abilities::active_replacements(state) {
	if def.event != ReplacementEvent::CopySpell || obj.controller != controller {
	continue;
	}
	count = match def.quantity_modification {
	Some(QuantityModification::Double) => count.saturating_mul(2),
	Some(QuantityModification::Plus { value }) => count.saturating_add(value as usize),
	Some(QuantityModification::Minus { value }) => count.saturating_sub(value as usize),
	// `Prevent` / unspecified is not a copy-count increase — leave as-is.
	Some(QuantityModification::Prevent) | None => count,
	};
	}
	count