feat: add bounded compatible session browser (#8)

Closes #8
2026-07-16 06:06:29 +02:00
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+# ADR 0006: bounded compatible session browser
+
+- Status: Accepted
+- Date: 2026-07-16
+- Tracking: #8
+
+## Decision
+
+The public list endpoint requires game, environment, and exact gameplay protocol.
+Region is optional, page size is 1–100, and callers may exclude sessions whose
+advisory current-player count has reached the advertised maximum. Lists contain
+public sessions only and only while both lease and authenticated host presence are
+fresh. Unlisted sessions never appear in a list; they may be retrieved directly by
+their 128-bit unguessable listing ID only when the caller also supplies the exact
+game, environment, and protocol scope.
+
+Results use ascending opaque listing ID as a deterministic keyset. A cursor carries
+the last ID plus every compatibility/filter field, a five-minute expiry, and an
+HMAC-SHA256 signature under a per-process key. Tampering, expiry, or reuse with a
+different tenant/protocol/region/full filter returns `InvalidRequest`. Restart
+rotates the key, matching the loss of ephemeral listings.
+
+Pagination is a bounded live view, not a database snapshot. A record that remains
+eligible and whose ID is greater than the cursor is returned exactly once. Records
+removed or made stale disappear immediately. A record created after a page whose ID
+sorts before that page's cursor is outside that traversal; callers refresh from the
+first page to discover new sessions. This avoids skips or duplicates among stable
+eligible records without retaining per-browser snapshot state.
+
+The store reads at most page size plus one record. The service serializes against
+the 256 KiB response ceiling and shortens a page before returning it when metadata
+makes the requested count too large. A continuation cursor is emitted whenever an
+extra or byte-trimmed record remains. All cursor, page, metadata, property, scalar,
+and collection sizes are bounded before untrusted allocation can grow without a
+ceiling.
+
+Browser DTOs are fresh copies containing only opaque listing ID, exact compatibility,
+region, visibility/trust presentation, advisory capacity, build/display labels, and
+policy-validated string metadata. They contain no observed endpoint, lease,
+capability, ticket, credential fingerprint, derivation salt, principal subject, or
+store key. Metadata is display text: JSON encoding escapes markup, but game UI must
+still render values as text and must never execute markup, interpret endpoints, or
+use metadata for authorization.
+
+## Consequences
+
+- Cross-game, cross-environment, incompatible, stale, revoked, expired, unlisted,
+  and optionally full sessions are removed before response construction.
+- Direct unlisted lookup is suitable for an out-of-band invite carrying the opaque
+  ID; human join codes remain future work and require their own bounded abuse model.
+- Host capacity remains advisory. The host makes the final admission decision.