Adds nthroot methods

[Pablo1Gustavo]

Add nthRoot() to BigInteger and BigDecimal

Summary

This PR adds nthRoot() methods to BigInteger and BigDecimal, providing
arbitrary-precision nth root computation with the full RoundingMode contract
used everywhere else in the library. Odd degrees accept negative inputs.

BigInteger::of('1267650600228229401496703205376')->nthRoot(100);
// => BigInteger('2')   — exact: 2^100

BigDecimal::of(2)->nthRoot(3, 20, RoundingMode::HalfUp);
// => BigDecimal('1.25992104989487316477')   — cube root of 2

BigInteger::of(-27)->nthRoot(3);
// => BigInteger('-3')   — negative input, odd degree

Motivation

brick/math already exposes sqrt() on both BigInteger and BigDecimal,
but callers who need a cube root, fourth root, or any higher degree root have
to reach for ad-hoc Newton iterations, or drop down to native float
arithmetic — losing the arbitrary precision that the library exists to
provide.

Real-world cases where this matters:

• Financial & statistical workloads — geometric means, CAGR
  ((final/initial)^(1/n) − 1), and volatility calculations commonly require
  non-square roots at precisions beyond IEEE-754 double.
• Cryptography & number theory — checking whether a large integer is a
  perfect kth power (e.g., Miller–Rabin preconditions, AKS primality) needs a
  floor integer nth root.
• Geometric modelling — cube roots and fifth roots show up in volume
  inversions, bezier parameterizations, and physics-engine mass
  distributions.
• Scientific computing — any formula involving x^(1/n) where x has
  more than 15 significant digits.

Today every one of these callers either rolls their own loop — a
well-known source of off-by-one bugs in the floor/ceiling boundary — or
accepts silent float rounding. Both are avoidable.

API

BigInteger::nthRoot()

public function nthRoot(
    int $n,
    RoundingMode $roundingMode = RoundingMode::Unnecessary,
): BigInteger;

Returns the integer nth root of $this, rounded according to
$roundingMode.

• $n must be ≥ 1.
• $this may be negative only when $n is odd.
• Default RoundingMode::Unnecessary throws RoundingNecessaryException if
  $this is not a perfect nth power — matching the library-wide contract
  that lossy operations require explicit opt-in.

BigDecimal::nthRoot()

public function nthRoot(
    int $n,
    int $scale,
    RoundingMode $roundingMode = RoundingMode::Unnecessary,
): BigDecimal;

Returns the nth root rounded to $scale decimal places.

• Same constraints on $n as above.
• $scale must be non-negative (as in BigDecimal::sqrt() and
  dividedBy()).
• Throws RoundingNecessaryException with distinct messages for "not
  exact" vs "exact but scale too small", mirroring sqrt().

Signature parity with sqrt()

	sqrt()	nthRoot()
BigInteger	sqrt(RoundingMode = Unnecessary)	nthRoot(int $n, RoundingMode = Unnecessary)
BigDecimal	sqrt(int $scale, RoundingMode = Unnecessary)	nthRoot(int $n, int $scale, RoundingMode = Unnecessary)
Negative input	rejected	rejected only for even $n
All 10 RoundingModes	✓	✓

Implementation

Two-layer split (matches sqrt())

┌────────────────────────────────────────────────────────────────┐
│  Public layer                                                  │
│  ├─ BigInteger::nthRoot(int $n, RoundingMode)                  │
│  │     · signs, identity fast-paths, rounding-mode logic       │
│  └─ BigDecimal::nthRoot(int $n, int $scale, RoundingMode)      │
│        · scale padding, one-extra-digit precision, rescale     │
│                         │                                      │
│                         ▼                                      │
│  Internal layer                                                │
│  └─ Calculator::nthRoot(string $n, int $k) → truncated root    │
│         ├─ GmpCalculator override → gmp_root()                 │
│         └─ shared fallback → Newton-Raphson                    │
└────────────────────────────────────────────────────────────────┘

Calculator layer

Calculator::nthRoot() returns the truncated-toward-zero integer nth root.
It has one backend-specific override:

• GMP → gmp_root(), with sign re-applied manually to freeze
  truncation-toward-zero semantics across PHP/GMP versions.

• BCMath & Native → shared Newton-Raphson fallback:

  x₀ = 10^(⌈digits(m)/k⌉)                                (overshoot)
  xᵢ₊₁ = ⌊((k−1)·xᵢ + ⌊m/xᵢ^(k−1)⌋) / k⌋                (recurrence)
  stop when xᵢ₊₁ ≥ xᵢ                                    (terminate)
  

  Starting strictly above the true root guarantees monotone convergence
  from above; the iterate stabilises when it can no longer decrease.

BigInteger::nthRoot() — rounding

After obtaining the truncated root r and computing r^n, the method
branches on $roundingMode:

• Down / Floor / Ceiling / Up — direct choice between r and
  the next step one unit further from zero.
• Half* — would normally require a midpoint comparison. But for
  consecutive integers r and r±1, the sum r^n + (r±1)^n is always
  odd, while 2·|value| is always even, so a midpoint tie is
  provably impossible. All five Half* modes therefore collapse to a
  single comparison: increment ⇔ 2·|value| > r^n + (r±1)^n.

BigDecimal::nthRoot() — scale handling

Given input scale s and target scale S:

1. Pad the value so s ≡ 0 (mod n) (append up to n−1 zeros).
2. Choose intermediateScale = max(S, s/n) + 1 (one guard digit).
3. Left-shift the value by n·intermediateScale − s zeros.
4. Take the integer nth root at the intermediate scale.
5. If the root isn't exact, pre-adjust for Up / Ceiling / Floor away-
   from-zero direction, and — since the true value is irrational —
   collapse all Half* modes to HalfUp (no midpoint tie is possible).
6. Rescale to S via DecimalHelper::scale().

All scale arithmetic goes through Safe::{add, sub, mul} so overflow
surfaces as IntegerOverflowException rather than silent float
corruption.

Testing

• 551 new PHPUnit assertions spread across 8 new test methods
  covering: identity cases (n=1), zero and unit inputs at all degrees,
  perfect powers for n ∈ {2, 3, 4, 5, 7, 100}, non-exact boundary
  cases exercising every RoundingMode, negative inputs with odd n,
  fractional inputs (inputScale % n ≠ 0), large scales (30), and every
  exception path.
• Rounding-mode equivalence expansion — each provider row
  automatically fans out to the modes that are mathematically
  equivalent for the given sign (e.g., positive-value Up ≡ Ceiling),
  so no branch is left unexercised.
• Cross-backend parity fuzz — random-tests.php now fuzzes
  nthRoot for k ∈ {2, 3, 4, 5, 7} on both positive and negative
  operands against all three backends, exiting on the first
  mismatch.
• PHPStan level 10 clean.

Test plan

• Full PHPUnit suite on CALCULATOR=Native (13,469 tests pass)
• Full PHPUnit suite on CALCULATOR=BCMath (13,469 tests pass)
• Full PHPUnit suite on CALCULATOR=BCMath with BCMATH_DEFAULT_SCALE=8
• vendor/bin/phpstan --no-progress — no errors at level 10
• BCMath ↔ Native parity fuzz on Calculator::nthRoot
• CI matrix: verify CALCULATOR=GMP on PHP 8.2–8.5 (64-bit and 32-bit)
• CI: random-tests.php for ≥ several million iterations across all three backends
• CI: tools/ecs coding-standard check

Backward compatibility

Purely additive: no existing API surface changes. Matches the
0.x.y bump convention documented in README.md for non-breaking
additions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.98%. Comparing base (6aef71a) to head (7cde0a6).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #113      +/-   ##
============================================
+ Coverage     98.86%   98.98%   +0.12%     
- Complexity      709      755      +46     
============================================
  Files            20       20              
  Lines          1759     1870     +111     
============================================
+ Hits           1739     1851     +112     
+ Misses           20       19       -1     

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