build(Makefile): enable parallel make -j gen with correct dependency graph (#22612)

`make gen` could not run with `-j` because inter-target dependency edges
were missing. Multiple recipes compile `coderd/rbac` (which includes
generated files like `object_gen.go`), and without explicit ordering,
parallel runs produced syntax errors from mid-write reads.

Three main changes:

**Dependency graph fixes** declare the compile-time chain through
`coderd/rbac` so that `object_gen.go` is written before anything that
imports it is compiled. The DB generation targets use a GNU Make 4.3+
grouped target (`&:`) so Make knows `generate.sh` co-produces
`querier.go`, `unique_constraint.go`, `dbmetrics`, and `dbauthz` in a
single invocation. `SKIP_DUMP_SQL=1` avoids re-entrant `make` inside
`generate.sh` when the Makefile already guarantees `dump.sql` is fresh.

**`scripts/atomicwrite` package** replaces `os.WriteFile` in all gen
scripts with a temp-file-in-same-dir + rename pattern, preventing
interrupted runs from leaving partial files.

**`.PRECIOUS` and shell atomic writes** protect git-tracked generated
files from Make's default delete-on-error behavior. Since these files
are committed, deletion is worse than staleness -- `git restore` is the
recovery path.

CI now runs `make -j --output-sync -B gen` (~32s, down from ~85s
serial).

| Scenario                          | Before             | After    |
|-----------------------------------|--------------------|----------|
| `make gen` (serial)               | 95s                | 95s      |
| `make -j gen` (parallel)          | race error         | **22s**  |
| CI `make -j --output-sync -B gen` | forced serial ~85s | **~32s** |

.github/workflows/ci.yaml

@@ -315,9 +315,7 @@ jobs:
           # Notifications require DB, we could start a DB instance here but
           # let's just restore for now.
           git checkout -- coderd/notifications/testdata/rendered-templates
-          # no `-j` flag as `make` fails with:
-          # coderd/rbac/object_gen.go:1:1: syntax error: package statement must be first
-          make --output-sync -B gen
+          make -j --output-sync -B gen
 
       - name: Check for unstaged files
         run: ./scripts/check_unstaged.sh