fix(tips): abbreviate tip presets that run past three digits

[bmc08gt]

Four chips share a row in the Send a Tip sheet, and a preset is a converted USD tier, so in a small-unit currency every tier is four digits or more. In ARS the three presets render $1,400 / $35,000 / $1,400,000, and Text's minimumScaleFactor(0.5) shrinks them to half size to fit the chip.

FiatAmount.formattedAbbreviated(maxDigits:) caps the figure at three digits. Under the first scale it is plain formatted(...); past that the value is scaled and suffixed K/M/B/T, keeping only the decimals the cap leaves room for.

Currency	Presets before	After
ARS	$1,400 $35,000 $1,400,000	$1.4K $35K $1.4M
COP	$97,500	$97.5K
VND	₫650,000	₫650K
IDR	400,000	400K
USD	$1 $5 $25	unchanged

It formats through the same NumberFormatter.fiat path as formatted, so the symbol, grouping, and the negative form (-$1.5K) come from one place.

The same rule as Android

code-payments/code-android-app#1381 does this on Android, and the first version here did not produce the same strings: it kept a decimal only while the scaled figure was a single digit, so it spent two digits of a three-digit budget — $1.2K against $1.23K, $12K against $12.3K, 26K against 25.5K for an ARS preset. Android's rule is the one that answers "three digits", so this takes it.

The implementation now mirrors Fiat.abbreviated(maxDigits:) step for step: round to maxDigits significant digits, pick the largest scale the rounded value clears, spend what is left on decimals. Rounding first is what makes 999,999 print $1M instead of $1,000K, and it is half-up on both sides. Two behaviours came along with it — the T scale, where 1e12 used to print $1,000B, and 999.99, which now carries up to $1K rather than printing five digits.

formattedDroppingZeroFraction() is Android's FormattingRule.Truncated: the fraction is dropped when the amount is whole and kept when it isn't. It formats the amounts under the first scale, and replaces the local customTitle helper the sheet had for the same job.

Two more things Android does that this now does:

• The custom "…" chip abbreviates as well. Android's slot is the same composable for presets and the custom amount, so it always did.
• Both chips hand VoiceOver the unabbreviated amount, matching Android's content description. The abbreviation is visual only.

One abbreviation rule, not two

CompactCurrencyFormatStyle — the market-cap and weekly-delta style in Currency Discovery — was a second abbreviation rule, built on ICU's .compactName with its own symbol and sign handling. It is now a two-line FormatStyle entry point onto formattedAbbreviated, for the Double figures SwiftUI formats inline.

Market caps therefore move with the shared rule, and being capped at three digits rather than two they gain a digit: $1,029,331 reads $1.03M rather than $1M, -12,400 reads -$12.4K. $10.5M no longer reads $10M — ICU compact notation rounds half-even, display rounding in this app is half-up.

That the row follows the tip chips is the intent, not a side effect. It is not width-constrained the way a chip is, and getting ICU's shape back would take a second digit budget that caps decimals without capping whole digits — maxDigits: 2 is not it, since it rounds $211K down to $210K. That is a third rule to keep in step with Android for the sake of one row.

The style's Int(value) truncation is kept, so a market cap still reads $200 rather than $200.17.

Two divergences this does not close

Both are older than either PR and need an Android-side call, so they are worth naming rather than papering over:

• ARS and COP symbols. Both platforms take the first single-character symbol the locale table offers. Apple's data gives ARS and COP a $; Android's does not, so the same peso preset reads $7.5K here and 7.5K there. IDR (bare) and VND (₫) agree.
• Market caps. Android keeps two abbreviation rules — Fiat.abbreviated for the tip chips and the uncapped Number.abbreviated for TokenMetricsRow, which always prints two decimals. A $690,272 cap reads $690K here and $690.27K there.

The parameterized cases in FiatAmountAbbreviatedTests follow Android's FiatTest — the scale boundaries, the decimal cap, the carry into the next scale, and the ARS/COP/VND/IDR/JPY rows — plus the negatives and the lower-cap case.