rpc/jsonrpc, node, execution/commitment: serve debug_executionWitness on minimal nodes via head-capture

[awskii]

A --prune.mode=minimal node keeps account/storage/code history but no commitment history, so the eager witness cache and debug_executionWitness are both hard-gated off — a minimal node can't serve execution witnesses at all.

This adds a head-capture mode that builds each head block's witness with commitment parent state read from an RO snapshot pinned at the parent (where commitment-latest already equals the parent trie) and plain state from the history a minimal node keeps. No commitment history is required or produced.

Changes

• --witness.cache.head-capture: build the last-N head witnesses against a rolling one-block-lag pinned parent snapshot, using a dual-tx reader (commitment-latest from the pinned parent tx and plain history from the committed tx). No code on the consensus commitment path.
• Serve cache-only: typed out-of-window on miss (never a history recompute); by-hash requests are canonical-checked so a reorged-out orphan is never served as canonical.
• Fail-closed: witnessRoot==parent, computedRoot==block, and stateless verify gate every cache insert.
• Cache is now a struct with a resident-bytes cap (--witness.cache.maxmb).

Tip-only and cache-only by design: after restart the cache is empty and re-warms forward, so the last ~N blocks are out-of-window until N new blocks pass.

Stacked on #22384.

[Copilot]

Pull request overview

Copilot reviewed 30 out of 30 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

rpc/jsonrpc/witness_cache.go:122

• residentBytes accounting uses int and is updated via c.residentBytes += size. On 32-bit builds (which the maxMB→bytes clamp explicitly calls out), a cache holding many large witnesses can overflow residentBytes (e.g., 96×~25MB > 2GB), making ResidentBytes()/byte-cap eviction logic incorrect. Use a wider type (int64) or saturating arithmetic when adding sizes to prevent overflow.

	if existed {
		c.residentBytes -= prev
	}
	c.entryBytes[hash] = size
	c.residentBytes += size