Consolidate equation discretization after v1

[ChrisRackauckas-Claude]

Ignore until reviewed by @ChrisRackauckas.

What changed and why

MethodOfLines v1 removed the selectable scalar discretization strategy, but the old source and test organization still presented array discretization as an alternative path. This follow-up deletes the standalone scalar-discretization file, moves its live pointwise helper into the unified equation-discretization source, and renames the array-specific source and test group around their current general role.

The documentation and problem-construction tests now state the solver boundary explicitly: discretize returns a DAEProblem intended for solve(prob), while explicit Runge–Kutta methods such as Tsit5() and SSPRK54() require symbolic_discretize, mtkcompile, and an ODEProblem. The explicit-path test now exercises Tsit5().

Reviewer judgment

The pointwise fallback machinery is intentionally retained. It is not a public or selectable strategy, but it remains active for unsupported slice patterns, boundary and corner equations, frame points, and stationary systems. Removing it would be a behavior change rather than dead-code cleanup.

Verification

• /home/crackauc/.juliaup/bin/julia +release -m Runic --check src test docs benchmark
  • exit code 0
• typos src test docs benchmark
  • exit code 0
• GROUP=Discretization /home/crackauc/.juliaup/bin/julia +release --project -e "using Pkg; Pkg.test()"
  • Equation discretization | 3107 / 3107 passed
  • Problem construction | 49 / 49 passed
  • Testing MethodOfLines tests passed
• GROUP=QA /home/crackauc/.juliaup/bin/julia +release --project -e "using Pkg; Pkg.test()"
  • QA | 12 passed, 6 repository-declared broken, 18 total
  • JET analyzed 403 definitions
  • Testing MethodOfLines tests passed
• /home/crackauc/.juliaup/bin/julia +release --project=docs docs/make.jl
  • exit code 0; doctests, examples, link checking, and rendering completed
• GROUP=Everything /home/crackauc/.juliaup/bin/julia +release --project -e "using Pkg; Pkg.test()"
  • renamed group passed again: 3107 equation-discretization assertions and 49 problem-construction assertions
  • full-suite QA: 12 passed, 6 repository-declared broken, 18 total; JET analyzed all 403 definitions
  • final output: Testing MethodOfLines tests passed
• git diff --check
  • exit code 0

Not verified locally

GPU, downstream, and Julia pre-release CI jobs were not run locally.

Links

• Preceding v1 PR: feat!: make array-form DAE discretization the v1 default #650