Make x a mutable integration cursor in stiff3 (intent(inout)) and propagate endpoint/interrupt semantics

[Copilot]

stiff3 previously treated both x and xend as intent(in), which prevented returning the actual integration position. This change makes x an in/out cursor so callers receive x == xend on successful completion, or the last accepted step when integration exits early.

• Solver API contract

  • Updated stiff3_auto, stiff3_work, and stiff3_core to take x as intent(inout).
  • Kept xend as intent(in).
  • Ensured all solver return paths write back the current x (normal completion and early callback interruption).

• Behavioral semantics

  • Successful integration now returns the terminal coordinate via x.
  • Early return from solout now returns the most recent accepted coordinate via x, preserving restartability from that point.

• Call-site compatibility updates

  • Updated examples/tests that passed parameter constants as the first integration bound to instead pass a mutable local variable initialized from the start value.
  • This affects benchmark-style examples and dense-output tests where x0 was previously immutable.

• Docs and usage updates

  • Updated API comments and README usage text to reflect mutable x semantics and clarify hmax default interpretation relative to the initial x.

• Targeted assertions

  • Extended tests to assert:
    • x == xend on normal completion.
    • x equals last accepted step on early exit.

real(wp) :: x, xend, y(n), h0, eps, w(n)

x    = 0.0_wp
xend = 1.0_wp
call stiff3(n, fun, x, y, xend, jac, h0, eps, w)

! On success: x == xend
! On early interrupt: x is the last accepted step