Share the SROCK tableau vectors instead of rebuilding them per cache (#4050)

SROCK2ConstantCache, TangXiaoSROCK2ConstantCache, KomBurSROCK2ConstantCache and
SROCKC2ConstantCache each built `recf` -- a 4476-element Vector{Float64}, 35 KiB
-- as an array literal inside the constructor, so every integrator allocated its
own copy of a compile-time constant table. SKSROCKConstantCache did the same with
199 BigFloat literals for `mc` and `mα`.

Nothing mutates these tables; the perform_step! methods only index them. Hoisting
them to module-level consts lets every cache share one copy. `convert(Vector{T},
...)` returns its argument unchanged when the element type already matches, so
Float64 solves share the table and other element types convert exactly as the
struct constructor did before.

This is not only a test-suite concern: an SDE solution retains its cache through
the `interp` field, so every retained trajectory of an SROCK ensemble was
carrying its own 35 KiB tableau.

Per-solution retention in an ensemble (2000 trajectories, scalar linear SDE,
save_everystep = false):

  SROCKC2 iip   38.72 -> 3.72 KiB     SROCK2 iip   40.63 -> 4.15 KiB
  SROCKC2 oop   37.59 -> 2.59 KiB     SROCK2 oop   39.44 -> 2.96 KiB
  SimplifiedEM (control, untouched)    2.25 -> 2.25 KiB

Allocation per cache construction:

  SROCK2          39368 -> 1760 bytes      SROCKC2        37208 -> 1296
  TangXiaoSROCK2  39192 -> 1616 bytes      KomBurSROCK2   40224 -> 1296
  SKSROCK         45024 -> 3488 bytes

SKSROCK keeps the allocation win (398 BigFloats are no longer constructed per
cache) but not the sharing one, since converting BigFloat to the cache element
type necessarily allocates.

The change is pure code motion. Verified:

  - The multiset of all 19329 numeric literals in the file is unchanged.
  - `git diff -w` is 382 lines; the rest of the diff is the 4-space de-indent
    from moving the literals out of the function bodies.
  - Solver output is bitwise identical on 32 cases -- SROCK1, SROCK2, SROCKEM,
    SKSROCK, SROCKC2, TangXiaoSROCK2 (versions 1 and 5) and KomBurSROCK2, each
    in-place and out-of-place, at dt = 1/2^5 and 1/2^7, comparing every saved
    state with `==`.
  - GROUP=Core tests pass (2m19s, exit 0), Runic clean.



Claude-Session: https://claude.ai/code/session_013WW2jGeoWZVZu7AiscUNSz

Co-authored-by: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: 3 people