feat(audit): Batch 10 — CC conventions (shared line-buffer, cleanError)

The contained consistency consolidations; the sweeping ones are deferred with
their standard documented (they're the audit's own post-1.0 refactors, and
several would be reckless to do unattended).

Landed:
- src/main/lib/lineBuffer.ts (createLineBuffer): one newline line-splitter for
  streamed child-process output, replacing the duplicated buffer loop in
  download.ts and terminal.ts (CC3/CC4/SIMP5). Pure + unit-tested
  (test/lineBuffer.test.ts, 7 cases: partial lines, CRLF, empty lines, flush,
  multi-line chunks). download.ts keeps byte-identical marker parsing and no
  close-flush (yt-dlp template lines are always newline-terminated).
- ytdlp.ts: getYtdlpVersion/updateYtdlp now run stderr through cleanError, the
  same `cleanError(r.stderr) || r.error?.message || ''` idiom already used by
  download/probe/indexer (CC6) — a failed check shows the trailing error line.
- CL6 resolved: the one real redundancy was L15; clearDir/openFile/showInFolder
  are deliberate view-model wrappers (CC13), left as-is.

Deferred with documented standard + rationale (CODE-AUDIT.md): CC1 (breaking
persisted-key rename → migration), CC5+CL4 (security-critical updater
net.request, not live-verifiable here), CC7 (cosmetic arg-order), CC9 (zod dep),
CC10 (electron-store↔jsonStore split is deliberate for DPAPI), CC11 (config.ts
move), CC12 (file-tree reorg), CC13→L93, CC14→L142.

Verified: typecheck (node+web) + 275 tests + eslint + production build green;
touched files prettier-clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/terminal.ts

@@ -14,6 +14,7 @@ import { getYtdlpPath, getBinDir, getSystem32Path } from './binaries'
 import { getSettings } from './settings'
 import { ensureManagedYtdlp } from './ytdlp'
 import { parseExtraArgs } from './buildArgs'
+import { createLineBuffer } from './lib/lineBuffer'
 import { IpcChannels, type TerminalEvent, type TerminalRunResult } from '@shared/ipc'
 
 const active = new Map<string, ChildProcess>()
@@ -47,18 +48,12 @@ export function runTerminal(wc: WebContents, id: string, argsRaw: string): Termi
   active.set(id, child)
 
   // Buffer each stream and emit on newline boundaries (flush the remainder on close).
-  const buffers: Record<'stdout' | 'stderr', string> = { stdout: '', stderr: '' }
-  function feed(stream: 'stdout' | 'stderr', chunk: string): void {
-    buffers[stream] += chunk
-    let nl: number
-    while ((nl = buffers[stream].indexOf('\n')) >= 0) {
-      const line = buffers[stream].slice(0, nl).replace(/\r$/, '')
-      buffers[stream] = buffers[stream].slice(nl + 1)
-      send(wc, { type: 'output', id, line, stream })
-    }
+  const lineBufs = {
+    stdout: createLineBuffer((line) => send(wc, { type: 'output', id, line, stream: 'stdout' })),
+    stderr: createLineBuffer((line) => send(wc, { type: 'output', id, line, stream: 'stderr' }))
   }
-  child.stdout?.on('data', (c: Buffer) => feed('stdout', c.toString()))
-  child.stderr?.on('data', (c: Buffer) => feed('stderr', c.toString()))
+  child.stdout?.on('data', (c: Buffer) => lineBufs.stdout.push(c.toString()))
+  child.stderr?.on('data', (c: Buffer) => lineBufs.stderr.push(c.toString()))
 
   let settled = false
   child.on('error', (err) => {
@@ -72,9 +67,8 @@ export function runTerminal(wc: WebContents, id: string, argsRaw: string): Termi
     settled = true
     active.delete(id)
     // Flush any trailing partial lines that never hit a newline.
-    for (const stream of ['stdout', 'stderr'] as const) {
-      if (buffers[stream]) send(wc, { type: 'output', id, line: buffers[stream], stream })
-    }
+    lineBufs.stdout.flush()
+    lineBufs.stderr.flush()
     send(wc, { type: 'done', id, code })
   })