PIP-50: align the spec with the reference implementation

[carbonflake]

Summary

This revises my Draft of PIP-50 after writing a full implementation in the node.
Building and testing it showed where the spec was ambiguous or unsafe.
The intent is unchanged: one live, deposit-backed commitment slot per account.

Status stays Draft. The node pull request will stay a draft until this PIP is Accepted.

What changed vs the merged revision

Topic	Before	After
UpdatedAt*	Refreshed by every Set	Moves only when RootHash, ManifestURI or AnchorType changes; a deposit top-up keeps the attestation date
Anchor pool	"Sized like Transfer"	10% of MaxSize, like BatchTransfer; anchors placed last in a block
JSON bytes	hex in JSON-RPC	base64 (like every protobuf bytes field); int64/uint32 encoding and BigInt note for locked_deposit
ZMQ anchor_info	Height little-endian	Big-endian like every other topic; exact frame layout, sizes and sequence gap rule
GetAnchor	Unspecified for validator/treasury	found = false; only a malformed address is an error
ListAnchors	Scan of all accounts	Local in-memory index, rebuilt at startup (still non-consensus)
Activation	75% implied as a rule	Clarified as the PIP-51 proposer rule, not a validation rule, and why
PAC-ANCHOR-1	simplemerkle (duplicates last node, no leaf/node separation)	RFC 9162 tree with BLAKE2b-256, name-bound leaves, size-bound root, proof algorithm, JSON proof document, test vectors
Test 21 / 22	Refresh on every Set; trailing bytes	Match the new date rule; forgery fails on signature

The PAC-ANCHOR-1 change matters most for apps: with the Bitcoin-style tree,
[a,b,c] and [a,b,c,c] share a root and an inner node can be proven as a 64-byte file.

Implementation status

The reference implementation covers the whole spec: payload, execution, pool,
activation at protocol version 5, gRPC / JSON-RPC / HTTP / ZMQ, wallet, and a
util/anchorprofile package for PAC-ANCHOR-1. Tests include lifecycle, two-node
consensus, restarts, fuzzing and an end-to-end run on four nodes; go test -race
and the deadlock job pass.

PR Checklist

• I have read and followed the proposal guidelines in PIP-1.
• This replaces PIPs/pip-50.md in place (same number, same author).