feat: `dot watch` — btop-style live block & event observer

Part of #226 (live chain observation). Build after `query --follow` proves the subscription plumbing.

What

A terminal-native live view of a chain — the `btop` for Polkadot:

dot watch                  # default chain
dot polkadot.watch         # explicit chain

Follows finalized (and/or new) heads and renders, as blocks arrive:

• block number + hash
• block author / validator (where derivable)
• extrinsic count (and optionally weight/size)
• a decoded live event tail (most recent N events, pallet.Event with decoded fields)

Why

This is the flagship slice of the streaming capability gap (#226). It gives the currently-blocked subscription RPC methods in `src/commands/rpc.ts` (`chain_subscribeFinalizedHeads`, `chain_subscribeNewHeads`, `state_subscribeStorage`) a real home, and it's differentiated — no good terminal-native Polkadot observer exists (Subscan / PJS-apps are web).

Approach

• papi observable subscription to finalized/new heads (flag to choose; default finalized).
• For each block: fetch events, decode with the existing post-tx event decoder in `tx.ts` (papi view-builder → `{ type, value }`).
• Render with the existing ANSI color infra in `src/core/output.ts`; reuse `Spinner`/redraw patterns.
• Run until SIGINT; clean teardown of subscription + client.

Output contract

• TTY: live-updating dashboard (in-place redraw of the header + scrolling event tail).
• `--json` / non-TTY: newline-delimited JSON, one object per block, drain-safe via `writeStdout`. Depends on #162 / #163 (valid JSON contract).

Open questions

• New-heads vs finalized default (lean: finalized, with `--new-heads` opt-in).
• Author derivation: how reliably can we resolve block author across relay/parachains? May ship without author in v1 and add later.
• Event filtering (`--pallet Balances`, `--event Transfer`) — likely a fast follow, not v1.
• No TUI lib currently in the repo (no blessed/ink); decide whether plain ANSI redraw is enough (preferred — keeps deps lean) or a lib is warranted.

Verification

• `dot polkadot.watch` renders a live header + event tail that advances ~every block.
• `--json` emits one parseable JSON object per block.
• Ctrl-C exits 0 and closes the WS connection.

Start with #226 and the subscription methods in src/commands/rpc.ts, then read the post-tx event decoder in tx.ts and ANSI output and redraw utilities in src/core/output.ts. Confirm the subscription plumbing before deciding the finalized/new-heads behavior and rendering approach. Done means dot polkadot.watch advances a live event tail, --json emits one parseable object per block, and Ctrl-C exits 0 while closing the WS connection.