fix(credentials-local): one operation chain, read-modify-write under the shared writer lock, and a quote-aware line editor

Review round three, credentials half. dsh-atomic-write grows the
cross-process writer-lock primitive (withFileLock: wx sentinel, bounded
backoff, stale takeover via onStaleBreak, deadline failure) plus a dirMode
option, and settings-local migrates its private copy to it; both providers
now create harness-home directories 0700.

credentials-local reuses the reviewed settings-local shape: watcher
reloads and line edits share one settled operation chain; every write
re-reads the document under the lock and publishes unobserved external
entries before editing, so an edit inside the debounce window (or another
process's write) can never be overwritten; the watcher's ready signal
queues one reconcile closing the startup gap.

The line editor is now physical-line aware: continuation lines of a
quoted multi-line value are never mistaken for assignments, untouched
lines keep their exact bytes (CRLF included), an edited line keeps its
own terminator, and appends use the document's dominant ending. A
multi-line entry reports writable: false, matching what set() would do.

The Credentials base class owns a contained notifyUpdated fan-out:
providers publish only after the commit, every listener runs, sync throws
and async rejections are logged without failing the committed write, and
INVARIANT-coded failures rethrow after the fan-out.

packages/credentials/credentials-local/tests/watcher.spec.ts

@@ -151,6 +151,7 @@ describe('watcher pipeline', () => {
     await fiber.dispose()
     disposed = true
     instance!.watcher.emit('all', 'change', path)
+    instance!.watcher.emit('ready')
     await new Promise(resolve => setTimeout(resolve, 100))
     expect(postDisposeCommits).toBe(0)
   })
@@ -204,4 +205,19 @@ describe('watcher pipeline', () => {
     await new Promise(resolve => setTimeout(resolve, 50))
     expect(await ctx.credentials.resolve(KEY)).toBeUndefined()
   })
+
+  it('reconciles at watcher ready so a change during setup is not missed', async () => {
+    const dir = await tempDir()
+    const path = join(dir, '.env')
+    await writeFile(path, `${KEY}=a\n`)
+    const ctx = await boot({ path, debounceMs: 5 })
+    // Written after the initial load but before the watcher became active:
+    // no 'all' event will ever fire for it.
+    await writeFile(path, `${KEY}=written-before-ready\n`)
+    const [instance] = await fakeInstances()
+    instance!.watcher.emit('ready')
+    await vi.waitFor(async () => {
+      expect(await ctx.credentials.resolve(KEY)).toEqual({ value: 'written-before-ready', source: 'file' })
+    })
+  })
 })