chore: delete the 25,984-line dead duplicate under examples/src

[physics515]

crates/mummu/examples/src/ was a stale copy of crates/mummu/src/, added by
commit 769d218 ("stuff", 2026-08-31) outside the PR process. Cargo never built
it, and it had already diverged from the real source — so it was pure weight that
silently polluted every grep/rg over the crate. It cost the 2026-09-07 nightly
a wrong read of where TensorSnapshot is used, and doubled the apparent site
count when auditing HybridKv and ParamSrc.

40 files, 25,984 deletions.

Proof the delete is inert

Three independent checks, because deleting 26k lines deserves more than "it looked
unused":

1. No main.rs anywhere under the tree — Cargo's example auto-discovery only
   promotes an examples/ subdirectory to a target when it contains one, so this
   was never a build target.
2. No [[example]] entry in any Cargo.toml (auto-discovery only), and no
   include! or #[path] reference to the tree from outside it.
3. cargo metadata reports the same 30 example targets before and after the
   deletion, none of them named src.

Verification

Gate	Result
cargo check --workspace --all-targets	EXIT=0 — 0 errors, 0 warnings
cargo metadata example targets	30 before, 30 after

--all-targets is the precise instrument here: it type-checks every lib, bin,
test, bench and example, so any surviving reference to the deleted files would
have failed the build. It ran cold, compiling the full burn/cubecl/Tauri graph.

The full cargo test --workspace suite was green at the parent commit e7e430c
(481 passed / 0 failed / 76 ignored across 48 binaries) and is re-running on this
branch; since this commit deletes only files that nothing compiles, no test
behaviour changes.

Closes the [ ] filed in ROADMAP P0 by the 2026-09-07 nightly, now ticked with
the proof recorded inline.

🤖 Generated with Claude Code