Bound the Link relay's resources, attachments and closures

[SaladDay]

Core will embed one Link relay (cutover PR2), and every sandbox and agent host will dial it. The relay bounded only per-link streams, pending renewals and queued answers. Serve peers, attachments and pending AttachmentClosed events grew without limit.

Change (internal/sandboxlink/relay):

• One counter mechanism, checked under the relay lock before the Authority is called, refuses admission beyond capacity with LimitExceeded and leaves no state.
• Resources (4096): the per-resource maps become one record that holds its slot while the relay holds anything for it (serve peer, Hello being decided, attachment or unwritten event). A reconnect reuses it; one serve link per resource bounds serve links too.
• Attachments (16384): an Open of an unknown attachment reserves a reference-counted slot that covers the attachment and its later AttachmentClosed events, so pending events are bounded by construction and never dropped for capacity.
• Attach links (4096): same counter.
• Values are constants, sized from measured idle cost for one Core running at most 1024 executions (under 500 MiB at capacity).

Behavior changes:

• RevokeResource discards events it could not deliver: the authority is withdrawn first, so that generation can never reconnect to read them.
• The relay forgets a resource's generation once it holds nothing for it; the Authority alone then refuses older generations.
• An Open whose attachment closes while the Authority decides fails with LeaseExpired instead of recreating it.

The wire does not change. docs/sandbox-link-protocol.md (+ zh) states the limits once and updates admission order, closures, staleness, the LimitExceeded row and Verification.

Checks: go test ./internal/sandboxlink/... (with fuzz seeds), go test -race -count=50 ./internal/sandboxlink/relay/, vet, gofmt, darwin/windows builds, make check-names check-docs check-ci.

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