Implement economy-transition-v6 in the C++20 kernel and retire version four's codec

[kaikisegfault]

Closes #177. Requirement 10 of docs/project/first-goal.md, for the byte and derivation surface.

What this delivers

include/protocol/v6/economy.hpp, nine sources under src/v6/, and five test translation units checked against test-vectors/economy-transition-v6.txt. It is a codec plus the pure derivations and performs no state transition — every entry point is a function of its arguments:

• the envelope, its two authorization schemes, the fourteen bodies and the five retired numbers;
• the admission shape rules the specification lists at step 1;
• the six HUB messages;
• the escrow and signer derivations;
• the security posture: slot_of, the confirmation predicate, and the relaxation predicate;
• the fourteen state entry encodings and the two retired ones;
• the economy tree, the version-six state root, genesis, chain identity, the receipt;
• the bounded mint walk and the verified-user arithmetic.

Version four's codec is removed rather than kept beside it

The kernel was compiling exactly one economy contract and it was the one already known to have no conforming implementation — version four's kind 11 names an identity the transaction never carries, which is why versions five and six exist.

Every Python model and vector file is retained: a model plus its vectors is the record of what the hosted matrix verified, and tools/economy-transition-v4-vectors/ still verifies its 441. A codec records nothing. ADR 0046 states the decision and the two rejected alternatives.

The shared account derivation

H(D("protocol-stack:v1:account") || 0x01 || pk) is now defined once in src/v1/account.hpp and used by both version one's admission path and version six's signer_id, rather than copied. The test checks it against test-vectors/protocol-primitives-v1.txt's recorded account_id — a third source, not a second restatement.

What it establishes, and what it does not

Of ADR 0045's four derived rules a codec can reach one, and it reaches it. NOTHING_TO_MINT is the empty walk range rather than the literal equality, so a mark can never decrease. DEBIT_OVERFLOW's position in the envelope order, the unrequested-confirmation refusal, and the cycle-assignment prologue need a ledger this does not have and are the next slice.

One of the three is pinned from the admitting side even so, and a probe is why. A mutation making the codec refuse a mint carrying a nonzero confirmation field — the rule version six's text literally states, at admission, under a code the result space does not contain — passed. Nothing in the test or in either accepted vector file noticed an implementation stricter than the contract can be. The test now requires such a mint to be admitted.

Evidence

Three checks reach a third source rather than a second opinion of the version-six file: the kind-1 identity and the signer derivation against protocol-primitives-v1.txt, the accounts tree against the same, and the two cycle-assignment records against economy-transition-v3.txt, because version six's settlement is version three's imported rather than reimplemented.

The populated economy root is what makes this more than a width check. The 44-entry fixture covers all fourteen assigned entry kinds, so one recorded root constrains every value encoding at once. Two probes swapping adjacent same-width fields — signer_count with exempt_slot_mask, and next_escrow_index with escrow_count — failed there and nowhere else.

Nineteen mutation probes fail closed, including the escrow label, the version-one account octet, the state-root schema version (a number, which the handoff warned would not appear in a search for v4), the RFC 9162 split replaced by a halving, the bitmap packed least-significant-bit first, a retired kind given a width, the scheme rule dropped, and genesis admitting an account.

Nothing accepted was edited. The v6 specification gains a status line and an evidence pointer and no rule; the v5 specification gains a corrected sentence about where the kernel's codec lives.

Verification

Local: the codec and all five test units compile clean under -Wall -Wextra -Wpedantic -Werror and pass; the version-one admission, execution, genesis, commitment, and block tests pass after the shared-derivation change; tests/tools/test_registration_test.py passes. The full compiler and sanitizer matrix is this PR's gate.