test: check formatting against an independent rendering

[thedavidmeister]

Closes #372

Formatting tests checked format against parse (round trip plus format(parse(s)) == s), against itself ("-" + format(x) == format(-x), two representations agree), or only checked length and some characters. A formatter that is wrong in the same way in every place passes all of these.

expectedFormat(c, e, scientific) builds the expected string from the OZ Strings.toString(|c|) digits. It does not call the formatter:

• plain: trailing zeros are stripped, then the decimal point goes e places from the right.
• scientific: d.ddd, followed by e and the leading digit's exponent (e + digits - 1). The e part is left out when that exponent is 0. _toScientific's NatSpec now states that rule.

testExpectedFormatOracle checks the oracle against hand-written strings.

Per the #376 ruling (b), the formatter's int32 and int224 refusal guards are to be dropped, so this PR asserts no guard revert. Only the documented ±1000 plain-mode cap is asserted.

Changes:

• Every test test: formatting is only checked against parse, never against an independent rendering #372 lists now asserts exact equality with the oracle. That includes the round trips (also for the re-parsed Float's own representation), the negative-sign test, the canonical-across-representations test, the length-only tests, the negative cap ("0." + 999 zeros + "1") and 1e67 ("1" + 67 zeros).
• testFormatNonScientificOutputShape is removed. Its shape checks are implied by exact equality, and testFormatNonScientificSucceedsForNonPositiveExponents now asserts the oracle over the same domain.
• New testFormatScientificFullExponentDomain covers every int224 coefficient and int32 exponent and asserts the oracle. Inputs whose leading digit exponent is above int32.max are skipped while src still refuses them.
• New testFormatNonScientificPositiveExponentBoundary covers e = 1..67, both signs: floor(int224.max / 10^e) must render as the oracle.
• _toScientific NatSpec: the eN part is omitted when N is zero.

The pre-existing guard-revert tests on main (testFormatScientificDisplayExponentOverflowReverts, testFormatScientificRevertsNearPositiveInt32Limit, testFormatNonScientificRevertsOnLongPositiveExp, the revert branch of testFormatParseRoundTripNonScientificSafePosExp, testFormatNonScientificLargePositiveExponent, testFormatNonScientificLargeCoefficientLargeExponent) are left for the #376 change that drops the guards.

QA

• Verified with mutation-probe (adversarial-mutation-test skill): one 39-mutant file over src/lib/format/LibFormatDecimalFloat.sol, run against a fresh clone of main 51a3451 and against this branch 02e2156. Suite: forge test --match-path test/src/lib/format/*, FOUNDRY_FUZZ_SEED=0x1. Main kills 34/39, this branch kills 36/39.
• Killed on this branch and survived on main: "e" -> "e+" for 1000 < displayExponent < 1e9 (killed by the oracle-equality fuzzes; main survives because the string still parses to the same value), and the int32 lower guard tightened by 100.
• Survive on both: exponent > MAX_NON_SCIENTIFIC_EXPONENT -> > MAX + 1 (equivalent: any exponent >= 68 already reverts with the same UnformatableExponent(exponent)), uExp >= 68 -> >= 69, and limit /= 10 -> /= 9. The last two only loosen the positive-exponent refusal guard that test: format refusal rules are copied from undocumented code guards #376 drops. They no longer fail here because the revert assertions were removed.
• The other 34 mutants (zero, mode switch, scale and exponent off-by-ones, sign handling, fraction leading zeros, scientific trailing-zero strip including keeping a 0 above 1e40, point characters, e0 rule, plain trailing-zero strip, point placement, leading zeros, cap lower bound) are killed on both.
• Oracle: expectedFormat takes the Strings.toString(|c|) digits and places them by the documented plain and scientific rules. It never calls the formatter, parse or maximizeFull, and is pinned by testExpectedFormatOracle against hand-written strings. The fuzz found no disagreement between the formatter and the oracle.
• Category check: test: formatting is only checked against parse, never against an independent rendering #372 asks for (a) round trips and canonical checks, (b) metamorphic-only checks and (c) no-value or partial-value checks to get an independent oracle. All three are covered. (a) RoundTripNonNegative, RoundTripNegative, ScientificFullDomain, NonScientificNegExpFullDomain, the SafePosExp success branch and AtIntDigitBoundary now assert the oracle. (b) CanonicalAcrossRepresentations and the - check now assert the oracle; OutputShape is removed because the oracle assertion covers it. (c) SucceedsForNonPositiveExponents and ScientificNegativeBoundaryDoesNotRevert now assert the oracle, and ExponentAtNegativeCap asserts the full string.

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[linear]

RAI-3156