Add seekable() to mmap.mmap (#8405)

* Allow mmap-backed readers to identify seek support

mmap already supports seek() and tell(); expose seekable() to match CPython.

Confidence: high
Scope-risk: narrow
Tested: test_mmap, stdlib_mmap snippet, clippy, prek, extra_tests
Not-tested: full workspace test blocked by local rustpython-capi SIGSEGV
Assisted-by: Codex:gpt-5.6-sol

* Keep mmap regression test lint-clean

CI ruff check removes the redundant blank line and treats the auto-fix as a failure.

Constraint: CI runs prek with ruff check
Confidence: high
Scope-risk: narrow
Tested: prek run --all-files; cargo run -- extra_tests/snippets/stdlib_mmap.py
Assisted-by: Codex:gpt-5.6-sol

* Keep mmap seekability method conventional

The method is invoked at runtime through the Python wrapper, so const qualification provides no benefit.

Constraint: Python methods execute at runtime
Confidence: high
Scope-risk: narrow
Tested: prek run --all-files; cargo run -- extra_tests/snippets/stdlib_mmap.py
Assisted-by: Codex:gpt-5.6-sol

crates/stdlib/src/mmap.rs

@@ -1141,6 +1141,11 @@ mod mmap {
             self.pos()
         }
 
+        #[pymethod]
+        fn seekable(&self) -> bool {
+            true
+        }
+
         #[pymethod]
         fn write(&self, bytes: ArgBytesLike, vm: &VirtualMachine) -> PyResult<PyIntRef> {
             let pos = self.pos();

extra_tests/snippets/stdlib_mmap.py

@@ -0,0 +1,6 @@
+import mmap
+
+mapped = mmap.mmap(-1, 1)
+assert mapped.seekable()
+mapped.close()
+assert mapped.seekable()