refactor: convert mlxcel-core and mlxcel-surgery into workspace members (#15)

Closes #7.

Adds `[workspace]` section to root `Cargo.toml` with Layout A: the
root `mlxcel` crate is simultaneously the workspace root and a member,
alongside `mlxcel-core` and `mlxcel-surgery`. `cargo metadata`
confirms all three crates enumerate as workspace members.

`.github/dependabot.yml` collapsed from 3 separate cargo entries
(root, mlxcel-core, mlxcel-surgery) to 1 root-level entry — the
unified workspace lock makes per-directory entries redundant. The
`github-actions` entry is unchanged.

Pre-flight finding: per-crate Cargo.lock files referenced in #7
task list were already absent. Actual scope reduced to `[workspace]`
addition + dependabot.yml simplification. The Cargo.lock gained 7
mlxcel-core dev-dependencies (sha2 0.11, block-buffer 0.12,
const-oid 0.10.2, crypto-common 0.2.1, hybrid-array 0.4.12,
tracing-test 0.2.6, tracing-test-macro 0.2.6) that were previously
invisible to the root lock and are now properly unified — this is
what fixes the long-standing `cargo test -p mlxcel-core` from-root
dev-dep resolution failure.

Verified locally: `cargo deny check` clean, `cargo check --lib
-p mlxcel-core --no-default-features` clean. Self-hosted clippy +
test gate (added in #14) is queued at merge time — will run on
main after the runner clears the PR #14 cold-build backlog.

Author: inureyes

.github/dependabot.yml

@@ -3,7 +3,7 @@
 
 version: 2
 updates:
-  # Cargo (Rust) dependencies — root `mlxcel` binary crate.
+  # Cargo (Rust) dependencies — unified workspace lock (mlxcel, mlxcel-core, mlxcel-surgery).
   - package-ecosystem: "cargo"
     directory: "/"
     schedule:
@@ -28,58 +28,6 @@ updates:
       prefix: "deps"
       include: "scope"
 
-  # Cargo (Rust) dependencies — `mlxcel-core` library crate (separate Cargo.lock).
-  - package-ecosystem: "cargo"
-    directory: "/src/lib/mlxcel-core"
-    schedule:
-      interval: "weekly"
-      day: "monday"
-      time: "09:00"
-      timezone: "Asia/Seoul"
-    open-pull-requests-limit: 10
-    groups:
-      minor-and-patch:
-        patterns:
-          - "*"
-        update-types:
-          - "minor"
-          - "patch"
-    ignore:
-      - dependency-name: "*"
-        update-types: ["version-update:semver-major"]
-    labels:
-      - "type:dependency"
-      - "area:core"
-    commit-message:
-      prefix: "deps(core)"
-      include: "scope"
-
-  # Cargo (Rust) dependencies — `mlxcel-surgery` library crate.
-  - package-ecosystem: "cargo"
-    directory: "/src/lib/mlxcel-surgery"
-    schedule:
-      interval: "weekly"
-      day: "monday"
-      time: "09:00"
-      timezone: "Asia/Seoul"
-    open-pull-requests-limit: 5
-    groups:
-      minor-and-patch:
-        patterns:
-          - "*"
-        update-types:
-          - "minor"
-          - "patch"
-    ignore:
-      - dependency-name: "*"
-        update-types: ["version-update:semver-major"]
-    labels:
-      - "type:dependency"
-      - "area:surgery"
-    commit-message:
-      prefix: "deps(surgery)"
-      include: "scope"
-
   # GitHub Actions dependencies.
   - package-ecosystem: "github-actions"
     directory: "/"