refactor: rename dyaka-noir-token to x-token

[xgreenx]

Scrubs the dyaka branding from this repo by renaming the X login circuit:

• circuits/dyaka-noir-token → circuits/x-token; Nargo package dyaka_noir_token → x_token; authors ["Dyaka"] → ["libid"]
• References updated in scripts/gen-verifier.sh (usage text), .github/workflows/ci.yml (comments, the 9-tests note), README.md (circuits table, gen-verifier example, extraction paragraph), and the main.nr header comment
• The README "Historical note (provenance)" section is collapsed to a minimal factual note; its one remaining dyaka_noir_token mention is deliberate — v0.1.0 shipped the circuit under that name and auditors need the thread from the release assets to today's x_token
• In passing, the README extraction paragraph was stale (claimed the sources were still not nargo fmt-clean; commit 1638b0b formatted them and CI enforces nargo fmt --check) — it now states the actual policy

vk byte-identity (the critical constraint)

XHonkVerifier.sol in libid-contracts derives from this circuit's vk and is deployed on chain, so the rename must not move the vk. Proven locally with the pinned toolchain from toolchain.env (nargo 1.0.0-beta.20, bb 5.0.0-nightly.20260324, exact versions verified):

• Built at origin/main (pre-rename) and on this branch (post-rename, via the exact scripts/build.sh release path); bb write_vk --oracle_hash keccak both times
• cmp on vk and vk_hash: byte-identical — vk sha256 fe6a945bf161fac154633f3dac1b86bd077de2a9e13c33eb56559bc7e128ce8c, vk_hash 0x0ef6521315935086e2161c8343e6e402c97e16770e1db9d53955fe1ca8f86e67
• ACIR json bytecode and abi fields are identical pre/post; only source-derived metadata moves (file_map paths, the hash field, debug spans — the header comment and package name live there, and bb derives the vk from the bytecode alone)

The next release build plus the libid-contracts verifiers CI job will re-prove this independently before any downstream pin moves.

Validation (CI parity, run locally)

• nargo fmt --check: clean for both circuits
• nargo test: x_token 9/9 pass; jwt_email untouched (0 tests, vacuous as in CI)
• shellcheck scripts/*.sh: clean
• Full scripts/build.sh succeeds with the renamed directory (tarball naming derives from the circuit dir, so the next release ships libid-circuits-<version>-x-token.tar.gz)
• No committed artifact or manifest embeds the old name — artifacts are release-only by design

Follow-up (not in this PR)

Deliberately no release, no tag, no downstream bumps here. After merge: cut the next release (assets become …-x-token.tar.gz / package x_token.json), then bump the libid-contracts circuits pin and let its verifiers job byte-compare the committed XHonkVerifier.sol against the new release, then roll any claim-full asset references.

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