History view: SQLite storage, daemon/TUI split over Unix socket

Implements build-order step 3. Adds:

- internal/store: SQLite (WAL, single-writer) history table, raw
  request/response blobs plus metadata for the list view.
- internal/proxy: request/response capture wired into forward(). HTTP/1.1
  legs are captured byte-exact via a teeConn that records wire bytes as
  they're read, taken right after the message is fully drained (so no
  manual re-reading/replaying is needed - RoundTrip's own streaming does
  the draining). HTTP/2 legs (no meaningful "raw bytes" of their own -
  multiplexed, HPACK-compressed framing) are reconstructed instead, and
  marked as such in storage.
- internal/ipc: JSON-over-Unix-socket protocol between mitmuxd (owns the
  proxy and the DB) and any client - list/get for queries, subscribe for
  a live push stream of newly captured entries. Keeps the proxy engine
  independent of the UI, per the architecture sketch.
- cmd/mitmux: Bubble Tea TUI - a live-updating history table and a
  request/response detail view with raw bytes.

Two real bugs surfaced during testing and got fixed before commit:
1. http.Transport's HTTP/2 auto-dispatch does a literal *tls.Conn type
   assertion on the dialed connection; wrapping it in a capturing teeConn
   broke that silently, and HTTP/2 framing got parsed as HTTP/1.1 text.
   Fixed by dropping http.Transport for the upstream leg entirely in
   favor of an explicit per-protocol round trip (see PLAN.md stack note).
2. singleConnListener wrapped the client teeConn *inside* a
   closeSignalConn, so ConnContext's type assertion for it silently
   failed and HTTP/1.1 client-side capture never activated. Fixed the
   wrap order; verified via direct SQLite inspection that request_exact
   flips back to 1 and the stored bytes are genuinely wire-exact
   (preserved chunked-encoding framing, original header casing/order).

Verified live: plain HTTP, HTTPS H1.1, HTTPS H2, and a POST with a body,
checked against the raw stored bytes directly in SQLite; IPC list/get/
subscribe against a throwaway client; and the TUI driven end-to-end in a
tmux session (list, detail view, tab between request/response, live
update on a new request while sitting on the list).

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