fix(hermes): runner concurrency + bridge restart in talk

[rafeegnash]

Summary

Bundled fix for #20 (hermes Runner scanner data race + lost notifications) and #21 (clanker talk doesn't recover when the bridge crashes). They share the same plumbing — the dispatcher's done signal is exactly where bridge-death detection lives, so splitting them would create surface-area for the seam to drift.

#20 — Runner dispatcher

• Single dispatcher goroutine owns bufio.Scanner. No other goroutine touches it. (Pre-fix: call() and Prompt() both called Scan() on the shared scanner → race + cross-talk.)
• Responses route by ID via a per-request inbox map; notifications go to the currently-active prompt's sink.
• promptMu serialises Prompt() calls — the wire protocol doesn't tag notifications with a request ID, so overlapping prompts can't be demuxed safely.
• ErrBridgeExited + IsBridgeExitError so callers can distinguish "bridge died" from "you asked a bad question."
• Pending callers unblock when the dispatcher sees EOF; previously they hung forever.
• Prompt drains queued notifications before yielding the final event so deltas aren't reordered behind the final when the bridge sends them back-to-back.
• Panic recovery on the dispatcher and prompt-streamer goroutines.

#21 — Bridge restart in talk

• cmd/talk REPL retries one prompt after restarting the bridge on ErrBridgeExited.
• Sliding-window cap: 3 restarts per minute. If the bridge keeps crashing, the REPL surfaces the error and exits cleanly rather than burning CPU in a restart loop.
• Small ringBuffer captures last 4 KiB of bridge stderr — when the restart message surfaces it includes the trailing 3 lines (typically the Python traceback's ModuleNotFoundError: ... line) instead of just "bridge process exited."

Test plan

• go test -race -count=1 ./internal/hermes/... — passes
• go vet ./... — clean
• gofmt -d — clean
• New tests cover:
  • ID-routed responses don't cross
  • Notifications delivered in order, then final
  • Concurrent Prompt callers don't see each other's deltas
  • Bridge-death unblocks pending calls with ErrBridgeExited
  • 200-delta flood: notif sink drops without wedging the final response
  • IsBridgeExitError recognises wrapped + joined errors

Closes #20
Closes #21