remote/oidc: PKCE + ephemeral loopback receiver (T1, T2)

[platypii]

Chunk 1 of the multi-tenant OIDC client login (LLP 0046-0048), milestone 1 local primitives.

What

• src/core/remote/pkce.js - createPkcePair(), S256 challenge over stdlib crypto. Client's downstream PKCE leg (LLP 0046 D3).
• src/core/remote/loopback.js - startLoopbackReceiver({ state, timeoutMs }), single-shot 127.0.0.1:0 listener on /callback; resolves { code } on a state-matched callback, rejects on mismatch / error= / timeout. RFC 8252 ephemeral redirect (LLP 0046 D2).

Tests

test/core/remote-pkce.test.js and test/core/remote-loopback.test.js, 7 tests green. npm run typecheck clean.

Base is the oidc-client-login integration branch, not master.

🤖 Generated with Claude Code

[platypii]

Review (independent agent review)

Verdict: approve-with-nits — PKCE encoding is correct (32-byte verifier, S256 = base64url(SHA-256(verifier)), padding/+// stripped) and the loopback's core guarantees (state-CSRF check before reading the code, single-shot close on first /callback, timeout rejection) hold and pass reliably. Conventions clean (no semicolons, no em dashes, @import types).

Findings

• major — close() did not reject a pending waitForCode(). Calling close() while a waitForCode() promise was outstanding set settled and cleared the timeout but never called rejectCode, leaking a permanently-pending promise (a latent public-API hang).
• minor — shared post-listen error handler. After listen resolved, a later server error event hit the pre-listen branch (rejectStart, a no-op) and left a pending waitForCode() unsettled and the server unclosed.
• nits — test gaps: no close() test; no assertion that a non-/callback path 404s without consuming the single shot.

Addressed in this PR

Fixed both: close() now rejects an in-flight (or future) waitForCode() with "closed before a code arrived"; post-listen errors route through fail() (settle + close). Added a started flag, a close() test, and a non-/callback-probe test. 9 tests green, typecheck clean.

The state-mismatch-aborts-the-flow behavior is intentional and acceptable under the loopback-only threat model.