fix: avoid missed wakeups in rpc stream subscribe

[luchenqun]

Description

Fix the rpc stream subscription race that could miss the final wakeup in blocking reads.
Initialize subscription offsets from the current tail so subscribers only receive new items.
Capture the cond channel before releasing the read lock to avoid dropped broadcasts.

Verification

go test -count=20 -race -tags=test ./rpc/stream -run 'TestStream(ReadBlocking|Subscribe)'
go test -race -tags=test ./rpc/stream

[greptile-apps]

PR author is not in the allowed authors list.

[aljo242]

two concerns before merge:

1. stream.go:74: changing offset = -1 to offset = s.lastID() is a silent behavior change. any caller relying on replay-from-beginning semantics will silently stop receiving historical items. please document this in the changelog and confirm no existing Subscribe call sites depend on the old behavior.

2. no regression tests. the missed-wakeup race fix and the new offset behavior both need test coverage. the pr description describes manual verification steps but those do not prevent future regressions. please add a test that exercises concurrent Add + ReadBlocking under -race to pin the race fix, and a test for the new subscribe-from-tail behavior.

minor: Channel() in cond.go is exported but only used internally by ReadBlocking. unexport it.

[aljo242]

silent behavior change in Subscribe offset, missing regression tests, unnecessary export of Channel().