Raise PrecompileTools floor to 1.2.1

[ChrisRackauckas-Claude]

Please ignore this PR until it has been reviewed by @ChrisRackauckas.

What changed

Pure [compat] lower-bound bump:

• PrecompileTools = "1" → "1.2.1"

No library code, tests, or package version were changed.

Why

ExponentialUtilities requires LinearSolve = "4.1, 5". LinearSolve 4.1.0 declares PrecompileTools = "1.2", and LinearSolve 4.1+ also requires SciMLBase 2.148+ which itself needs PrecompileTools 1.2.1. The declared "1" floor lets 1.0.0–1.2.0 through; all of those are unsatisfiable against the LinearSolve 4.1+ constraint.

The source does import PrecompileTools: @compile_workload, @setup_workload.

Verification

Julia 1.10.11. Pkg.develop of ExponentialUtilities, then Pkg.add(name="PrecompileTools", version=...):

# 1.0.0 (old floor)
DEVELOP_OK
RESOLVE_FAIL PrecompileTools=1.0.0
Unsatisfiable requirements detected for package PrecompileTools [aea7be01]:
 ├─restricted to versions 1.0.0 by an explicit requirement
 └─restricted by compatibility requirements with LinearSolve to versions: 1.2.0-1.3.4 — no versions left
   └─restricted to versions 4.1.0-5 by ExponentialUtilities

# 1.0.1
DEVELOP_OK
RESOLVE_FAIL PrecompileTools=1.0.1
# LinearSolve 4.1+ requires PrecompileTools >= 1.2.0

# 1.2.0
DEVELOP_OK
RESOLVE_FAIL PrecompileTools=1.2.0
Unsatisfiable requirements detected for package SciMLBase:
 └─restricted by compatibility requirements with LinearSolve to versions: 2.148.0-3.47.0 — no versions left
   (PrecompileTools 1.2.0 only supports SciMLBase through 2.131)

# 1.2.1 (new floor)
DEVELOP_OK
RESOLVE_OK PrecompileTools=1.2.1
LOAD_OK ExponentialUtilities

Not verified: the full test suite, a live julia-downgrade-compat run, or GPU.