Cap Symbolics to v6 and fix test Manifolds/Manopt resolution

Three independent CI failures all stem from dependency churn, not code
regressions:

1. docs + Core/QA precompile: Symbolics v7 removed the `Symbolic` abstract
   type and collapsed array/scalar registered symbolics onto a single
   `BasicSymbolic{SymReal}`. That makes the package's `import Symbolics:
   Symbolic` fail and makes the dual scalar/vector `@register_symbolic`
   (e.g. `quad_over_lin`) plus the matrix-function registrations
   (`Base.log`/`inv`/`sqrt` on `Symbolics.Arr`) generate duplicate /
   `Base`-clobbering methods that abort precompilation. The package's
   matrix-atom registration model is not yet v7-compatible, so cap
   `Symbolics = "6"` / `SymbolicUtils = "3.1.2"` (the last-known-good set,
   matching registered v0.3.6) in the package, docs, and test envs.

2. Core/QA resolution: dependabot bumped test `Manifolds` to 0.11.26, but
   `OptimizationManopt` 1.3.2 only supports `Manifolds 0.10` /
   `ManifoldsBase 1`, and `Manopt 0.5.38` requires `ManifoldsBase 2`. Cap
   test `Manifolds = "0.10"` and `Manopt = "0.5.25"` so the env resolves
   (Manifolds 0.10.23, ManifoldsBase 1.2.0, Manopt 0.5.25, OptimizationManopt
   1.3.2).

3. Downgrade: `empty intersection between Symbolics@6 and project compat
   7.25.0-7` — the test env pinned Symbolics 7.25 while the package floor is
   6. Aligning both to 6 removes the conflict.

Verified locally on Julia 1.12 and 1.10: the test env resolves and
precompiles, `GROUP=Core` passes (alloc 17/17, dgp 28/28, interface 31/31,
lorentz 11/11, test 19 pass + 2 pre-existing broken), and all docs example
blocks run.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

Project.toml

@@ -34,6 +34,6 @@ PDMats = "0.11"
 PrecompileTools = "1"
 RecursiveArrayTools = "3, 4"
 StatsBase = "0.34"
-SymbolicUtils = "3.1.2, 4.33"
-Symbolics = "6, 7.25"
+SymbolicUtils = "3.1.2"
+Symbolics = "6"
 julia = "1.10"

docs/Project.toml

@@ -33,5 +33,5 @@ PDMats = "0.11"
 RecursiveArrayTools = "3, 4.3"
 StatsBase = "0.34"
 SymbolicAnalysis = "0.3.6"
-SymbolicUtils = "3, 4.33"
-Symbolics = "6, 7.25"
+SymbolicUtils = "3"
+Symbolics = "6"

test/Project.toml

@@ -21,13 +21,13 @@ AllocCheck = "0.2.4"
 Aqua = "0.8"
 ForwardDiff = "1.3.3"
 JET = "0.9,0.10,0.11"
-Manifolds = "0.11.26"
-Manopt = "0.5.38"
+Manifolds = "0.10"
+Manopt = "0.5.25"
 Optimization = "5.6.1"
 OptimizationBase = "5.1.3"
 OptimizationManopt = "1.3.2"
 PDMats = "0.11"
 SafeTestsets = "0.1.0"
 SciMLTesting = "1"
-Symbolics = "7.25.0"
+Symbolics = "6"
 Zygote = "0.7.10"