fix: cslib +typeChanged on the ReflGen.symmGen_symm deprecation

[Kha]

Adapts cslib to a deprecation that the downstream: update repo cslib merge
(29d3da3, the head of run 34133371360)
brought in.

cslib (noncritical, --wfail --iofail)

That merge rewrote Cslib/Foundations/Relation/Confluence.lean and deprecated

lemma ReflGen.symmGen_symm : ReflGen (SymmGen r) a b → ReflGen (SymmGen r) b a

in favour of Relation.ReflGen.stdSymm — mathlib's [Std.Symm r] : Std.Symm (ReflGen r)
instance (Mathlib/Logic/Relation.lean:367). Since lean4#14600 the deprecation linter
compares the replacement's type with the deprecated declaration's, and an instance is not
the lemma it replaces:

warning: Cslib/Foundations/Relation/Confluence.lean:420:2: The updated constant has a different type:
  ∀ {α : Type u_1} {r : α → α → Prop} [Std.Symm r], Std.Symm (ReflGen r)
instead of
  ∀ {α : Type u_1} {r : α → α → Prop} {a b : α}, ReflGen (SymmGen r) a b → ReflGen (SymmGen r) b a

cslib builds with --wfail, so the warning is a build failure. The difference is
intended — the instance is both the lemma's own proof and what callers are meant to use
via Std.Symm.symm — so this marks it with +typeChanged (syntax from lean4#14570).

Verified on nightly-2026-09-06: lake build --wfail --iofail (3211 jobs),
lake test --wfail --iofail (9092 jobs) and lake lint all pass for cslib. The test
and lint runs also had #50's one-line rwa hunk applied in the working copy, because
without it the build stops at Cslib.Computability.Languages.MyhillNerode before those
phases can run; the two hunks are in different files and independent. With this commit
alone, Cslib.Foundations.Relation.Confluence builds clean and MyhillNerode is the
only remaining logged failure.

Not fixed here — already covered by open PRs

cslib's other failure, Cslib.Computability.Languages.MyhillNerode under the
rwaSuggestion linter, is #50, still MERGEABLE and still applying. Both it and this PR
are needed for cslib to build.

reference-manual, the one critical repo in the red set, is #51, still MERGEABLE.
This run is the first in which CI actually built it — verso, verso-slides and
verso-web-components are all green again now that subverso is out of repos.toml, so
the skip chain that had hidden it since 2026-09-03 is gone — and it failed at exactly the
six Manual/BuildTools/Lake/Config.lean sites that #51 fills in, with
Manual.BuildTools.Lake.Config the only logged failure. Nothing added here.

[downstream-lean4]

Build report for fix: cslib +typeChanged on the ReflGen.symmGen_symm deprecation

Stayed red

Repo	Critical	Build	Test	Lint
reference-manual	✅	🟥 in 24s	⏭️	⏭️
cslib		🟥 in 8s	⏭️	⏭️

Stayed green

Repo	Critical	Build	Test	Lint
aesop	✅	✅ in 7s	✅ in 4s	⏭️
batteries	✅	✅ in 4s	✅ in 4s	✅ in 2s
import-graph	✅	✅ in 2s	✅ in 3s	⏭️
lean4-cli	✅	✅ in 1s	✅ in 0s	⏭️
mathlib4	✅	✅ in 183s	✅ in 41s	✅ in 90s
plausible	✅	✅ in 1s	✅ in 2s	⏭️
ProofWidgets4	✅	✅ in 3s	✅ in 1s	⏭️
quote4	✅	✅ in 2s	✅ in 1s	⏭️
BibtexQuery		✅ in 1s	⏭️	⏭️
comparator		✅ in 2s	⏭️	⏭️
doc-gen4		✅ in 6s	⏭️	⏭️
illuminate		✅ in 3s	✅ in 10s	⏭️
lean4-unicode-basic		✅ in 2s	⏭️	⏭️
lean4export		✅ in 0s	✅ in 8s	⏭️
LeanSearchClient		✅ in 1s	✅ in 0s	⏭️
leansqlite		✅ in 4s	✅ in 20s	⏭️
nerodia		✅ in 2s	✅ in 21s	⏭️
repl		✅ in 1s	✅ in 58s	⏭️
verso		✅ in 36s	✅ in 86s	⏭️
verso-slides		✅ in 50s	✅ in 6s	⏭️
verso-web-components		✅ in 30s	⏭️	⏭️

View run

[Kha]

Superseded by #54. The downstream: update repo cslib merge ffe4553 moved the @[deprecated Relation.ReflGen.stdSymm] attribute out of Confluence.lean into Cslib/Foundations/Relation/Basic.lean, which made this branch CONFLICTING. #54 carries the same +typeChanged at the new location, plus the second site (MJoin.refl) that the same merge added.