Add ICM (Independent Chip Model) calculator

[testtest126]

Summary

• ICM.equities(stacks:payouts:) — exact Malmuth-Harville tournament $EV via bitmask-memoized recursion over finishing-position probabilities (O(2ⁿ·n²), not literal n! enumeration). The one model in this codebase that's exact math rather than a hand-tuned percentage table.
• Validated against a published worked example (Wikipedia's "Independent Chip Model" article: 50/30/20 stacks, 70/30 payouts) — matched to the source's own rounding, and to 1e-9 against exact fractions independently re-derived by hand (shown in full in ai-docs/ICM.md so the arithmetic is checkable without running code). A second hand-derived 3-payout example exercises the recursion's 3rd-place branch, which the 2-payout published example never reaches.
• Sanity gates: equal stacks split equally, the 2-player closed form, total-equity-conservation across varied fields, and the "ICM tax" (chip leader's $/chip < short stack's $/chip).
• ICMRiskPremium.assess — an ICM-adjusted required-equity-to-call overlay for all-in confrontations (same "adjustment layer, not mutation" shape as BountyEquity), with every simplification disclosed in the doc comment and ai-docs/ICM.md: single full-stack all-in, busting = $0 (not a guaranteed min-cash — flagged as slightly overstating the true premium), no Future Game State, and otherStacks must be the complete remaining field.
• New ICM Calculator app view (stacks + payouts in, per-seat $EV out, live recompute — ICM.equities is pure/fast, no async dispatch needed), wired into StudyTool. Deliberately scoped to just the calculator; ICMRiskPremium is implemented and tested but has no dedicated screen yet (noted as a natural follow-up, not a gap).

Test plan

• swift test (PokerKit): 220/220 green (208 baseline + 12 new ICM tests), no lost tests.
• App build succeeds on simulator (iPhone 17, iOS 27.0, Xcode 27 beta).
• App UI test suite: 18/18 green (excludes BankrollTrackerUITests.testLogSessionUpdatesListAndSummary, previously confirmed pre-existing/flaky on a clean main checkout, unrelated to this or the prior PR).
• New ICMCalculatorUITests covering default-stacks equity summing to the prize pool, adding a player row, and invalid-stack validation.

ICM validation numbers (actual vs. published)

Wikipedia's worked example (stacks 50/30/20, payouts 70/30):

Seat	Published (≈)	This project (exact)
A	$45	$45.178571... (1265/28)
B	$32	$32.25 (129/4)
C	$22	$22.571429... (158/7)

Sum: exactly $100 in both the published figures (approximately) and this project's exact computation.

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