rust-toolchain.toml: add rust-analyzer component (pin regression fix)

The 1.95.0 pin (7171282) shipped components = [clippy, rustfmt],
profile = minimal. The M4 LSP acceptance suite
(tests/m4_acceptance.rs:1111 m4_5_rust_analyzer_initializes) spawns
`rust-analyzer`; its skip-guard only checks PATH presence. The
`rust-analyzer` on PATH is a rustup proxy shim — with the directory
override active it resolves against the pinned toolchain, finds no
rust-analyzer component, and the spawn dies before the LSP
handshake, so the test runs (doesn't skip) and panics "must observe
Initialized event". This was masked in CI before the pin because the
Test job died at compile (rope.rs:1076); fixing compile unmasked it.

A pin must ship a toolchain complete for the project's own test
surface. Adding `rust-analyzer` to components.

Verified under 1.95.0: `rust-analyzer --version` resolves;
m4_5_rust_analyzer_initializes passes (was the sole failure in the
ubuntu Test legs — CI log showed m4_acceptance 52 passed / 1 failed,
all other suites ok). rust-toolchain.toml-only; no code change.

Still independent / triage-pending (unchanged): macOS F9 nix
PeerCredentials (Test (macos-*)), M1/M4/M6 perf/fuzz gates.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

rust-toolchain.toml

@@ -6,5 +6,10 @@
 # silently red-lined CI under newer rolling-stable lints. Bump
 # deliberately (its own validated cycle), never implicitly.
 channel = "1.95.0"
-components = ["clippy", "rustfmt"]
+# rust-analyzer is required: the M4 LSP acceptance suite spawns it
+# (tests/m4_acceptance.rs). Without it the rustup proxy resolves
+# `rust-analyzer` against this pinned toolchain, finds no component,
+# and the spawn dies before the LSP handshake — pinning must ship a
+# toolchain complete for the project's own test surface.
+components = ["clippy", "rustfmt", "rust-analyzer"]
 profile = "minimal"