fix(interpreter): process substitution expands to /dev/fd/63

[chaliy]

Requested by Михайло · project thread

What changed

• <(cmd) / >(cmd) expand to bash's /dev/fd/63, /dev/fd/62, ... (highest free fd counting down from 63, skipping fds already open), restarting per command like bash.
• The paths live in a per-interpreter fd table (ProcSubFs, a wrapper over the interpreter's view of the filesystem). /dev/fd/N (N ≥ 3) is never passed to the shared VFS, so interpreters sharing one filesystem each have their own /dev/fd/63 (new TM-ISO-028).
• A substitution reads as its data, stats as a FIFO, and is gone after its command (cat /dev/fd/63 → No such file or directory).
• Removed the global PROC_SUB_COUNTER and the /dev/fd/proc_sub_<n> files written into the VFS.
• set -C no longer refuses > >(cmd); exec 3> >(cmd) no longer aborts the script on later writes.

Why

Level-up roadmap, rootfs/OS-shape step: closes the process-substitution-dev-fd bash-oracle gap while keeping tenants isolated in one process.

Before / After (verified)

bash-oracle: match 103 → 104, strict 101 → 102 / 106 (rebased on #2638; floor.txt raised to 104 102). With a coproc open, <(true) skips its fds like bash: 63 60 /dev/fd/62.

echo <(true) <(true) >(true)
diff <(printf 'a\nb\n') <(printf 'a\nc\n'); echo rc=$?
cat /dev/fd/63; echo rc=$?

	first line
main	/dev/fd/proc_sub_0 /dev/fd/proc_sub_1 /dev/fd/proc_sub_2
bash 5.2 / this branch	/dev/fd/63 /dev/fd/62 /dev/fd/61

The diff output, rc=1, and cat: /dev/fd/63: No such file or directory / rc=1 are identical between bash and this branch.

Risk

• Isolation tests: a recording filesystem shows no /dev/fd path reaching the shared FS; a second interpreter on the same FS can't read, see or write the first's /dev/fd/63; 4 parallel interpreters each read only their own data.
• Open substitutions are capped by max_file_descriptors. Stack for depth_32_no_stack_overflow is slightly below main.
• Remaining gaps (limitations.md): a substitution is always its own word (x<(true)); data is buffered, not a live pipe; exec 3> >(cmd) drops fd 3 writes.

Checklist

• Failing tests first (process_substitution_fd_tests.rs, 14 tests; two bashbox spec cases enabled), verified against bash 5.2
• fmt, clippy, integration, lib, terminal, CLI, bash_comparison, check-okf
• knowledge updated (threat-model TM-ISO-028, limitations, log)

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