Support 32-bit platforms and add a 32-bit CI lane via test_groups.toml

[ChrisRackauckas-Claude]

Supersedes #95. Same 32-bit source support, but the CI lane is declared through the matrix (test/test_groups.toml) instead of a hand-added tests.yml job — and it fixes a real gap that made #95's own suite error on 32-bit.

What changed

Source

• src/simd_ir.jl — the LLVM-IR SIMD kernels are typed for 64-bit lengths/pointers, so guard them behind @static if Sys.WORD_SIZE == 64 and add a portable scalar fallback _scalar_first backing every primitive on 32-bit. (From Support 32-bit platforms #95.)
• src/equality.jl — pass lengths as Int64 explicitly (from Support 32-bit platforms #95), and add a generic findfirstsortedequal fallback. Unlike findfirstequal, findfirstsortedequal had only the (Int64, DenseVector{Int64}) method. On 32-bit, native Int is Int32, so findfirstsortedequal(::Int32, ::Vector{Int32}) threw MethodError — 8385 such errors in the suite (this is why Support 32-bit platforms #95 as-is does not actually pass on 32-bit). The new method is a generic searchsortedfirst + equality post-check, mirroring findfirstequal's fallback, so it works for any sorted vector on any platform.

CI (the point of this PR)

Instead of #95's hand-added tests-32bit job in Tests.yml, add a "Core 32-bit" section to test/test_groups.toml:

["Core 32-bit"]
group = "Core"        # dispatch the Core body (folder resolves) on GROUP=Core
versions = ["1"]
os = ["ubuntu-latest"]
arch = "x86"

Tests.yml stays a thin caller of grouped-tests.yml@v1. This relies on the new arch axis and group alias added to the SciML/.github matrix generator in SciML/.github#113 — that PR must merge and v1 be retagged before the lane activates in CI.

Tests & version

• Added an explicit _scalar_first-vs-findfirst parity test (from Support 32-bit platforms #95) and a new generic-findfirstsortedequal fallback test (Int32/Int16/Float32), so both paths are exercised on 64-bit CI too.
• Minor version bump 3.0.3 → 3.1.0 (new public behavior: findfirstsortedequal now accepts any sorted vector).

Local verification (actually run, not inferred)

• 32-bit i686 Julia 1.11.9 (juliaup add 1.11~x86), GROUP=Core Pkg.test(): passes, 0 errors with the fix. Without the generic fallback it errored 8385 times — confirmed and root-caused.
• x64, final state incl. new testset: passes, 171915 tests.
• Both changed .jl files are Runic-clean.

Dependency

Blocked on SciML/.github#113 (arch axis + group alias). Until v1 includes it, the arch/group keys are inert and the existing 64-bit matrix is unchanged (verified: no-arch test_groups.toml output is byte-identical to before).

Please ignore until reviewed by @ChrisRackauckas.

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