zencore: CreateProc stdin pipes + BuildArgV quote stripping (#983)

Two related improvements to `CreateProc`:

### 1. Stdin pipe support
- Adds `StdinPipeHandles` + `CreateStdinPipe` alongside the existing `StdoutPipeHandles`, letting callers feed data into a child process's stdin.
- Platform-agnostic RAII (Windows `HANDLE` pair / POSIX `pipe()` fd pair) with the same semantics as the stdout pipe: the inherited end goes to the child, the non-inherited end stays with the parent, destructor closes both.
- `CreateProcOptions` gains a `StdinPipe*` field.
- On Windows, `CreateProcNormal` is reworked so stdin/stdout redirection handles all combinations (stdin + stdout, each alone, neither) uniformly. POSIX already supported arbitrary fd redirection and just needed to honor the new option.
- `zentest-appstub` gains a `-stdin_echo` mode that reads stdin to EOF and echoes it back (switching to binary mode on Windows so CRLF translation doesn't mangle bytes).
- `zenserver-test` gets a `server.process` / `stdin_pipe.*` test group that exercises launching a child with a stdin pipe, writing, closing the write end, and reading back the echoed data.

### 2. Shell-style quote stripping in `BuildArgV`
- Callers that build a single command-line string for `CreateProc` commonly wrap spacey paths in double quotes (e.g. `--tracefile="$path"`). The old `BuildArgV` only used quotes to suppress space-splitting and left the characters in the resulting argv element, so the spawned process saw literal `--tracefile="..."` and the value parser failed to open the quoted path.
- `BuildArgV` now compacts in place, dropping quote chars as it goes, matching shell semantics for paired double quotes.

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